PERLFUNC(1) Perl Programmers Reference Guide PERLFUNC(1)
NAME
perlfunc - Perl builtin functions
DESCRIPTION
The functions in this section can serve as terms in an expression. They fall into two major categories: list operators and named unary
operators. These differ in their precedence relationship with a following comma. (See the precedence table in perlop.) List operators
take more than one argument, while unary operators can never take more than one argument. Thus, a comma terminates the argument of a unary
operator, but merely separates the arguments of a list operator. A unary operator generally provides scalar context to its argument, while
a list operator may provide either scalar or list contexts for its arguments. If it does both, scalar arguments come first and list
argument follow, and there can only ever be one such list argument. For instance, splice() has three scalar arguments followed by a list,
whereas gethostbyname() has four scalar arguments.
In the syntax descriptions that follow, list operators that expect a list (and provide list context for elements of the list) are shown
with LIST as an argument. Such a list may consist of any combination of scalar arguments or list values; the list values will be included
in the list as if each individual element were interpolated at that point in the list, forming a longer single-dimensional list value.
Commas should separate literal elements of the LIST.
Any function in the list below may be used either with or without parentheses around its arguments. (The syntax descriptions omit the
parentheses.) If you use parentheses, the simple but occasionally surprising rule is this: It looks like a function, therefore it is a
function, and precedence doesn't matter. Otherwise it's a list operator or unary operator, and precedence does matter. Whitespace between
the function and left parenthesis doesn't count, so sometimes you need to be careful:
print 1+2+4; # Prints 7.
print(1+2) + 4; # Prints 3.
print (1+2)+4; # Also prints 3!
print +(1+2)+4; # Prints 7.
print ((1+2)+4); # Prints 7.
If you run Perl with the -w switch it can warn you about this. For example, the third line above produces:
print (...) interpreted as function at - line 1.
Useless use of integer addition in void context at - line 1.
A few functions take no arguments at all, and therefore work as neither unary nor list operators. These include such functions as "time"
and "endpwent". For example, "time+86_400" always means "time() + 86_400".
For functions that can be used in either a scalar or list context, nonabortive failure is generally indicated in scalar context by
returning the undefined value, and in list context by returning the empty list.
Remember the following important rule: There is no rule that relates the behavior of an expression in list context to its behavior in
scalar context, or vice versa. It might do two totally different things. Each operator and function decides which sort of value would be
most appropriate to return in scalar context. Some operators return the length of the list that would have been returned in list context.
Some operators return the first value in the list. Some operators return the last value in the list. Some operators return a count of
successful operations. In general, they do what you want, unless you want consistency.
A named array in scalar context is quite different from what would at first glance appear to be a list in scalar context. You can't get a
list like "(1,2,3)" into being in scalar context, because the compiler knows the context at compile time. It would generate the scalar
comma operator there, not the list construction version of the comma. That means it was never a list to start with.
In general, functions in Perl that serve as wrappers for system calls ("syscalls") of the same name (like chown(2), fork(2), closedir(2),
etc.) return true when they succeed and "undef" otherwise, as is usually mentioned in the descriptions below. This is different from the C
interfaces, which return "-1" on failure. Exceptions to this rule include "wait", "waitpid", and "syscall". System calls also set the
special $! variable on failure. Other functions do not, except accidentally.
Extension modules can also hook into the Perl parser to define new kinds of keyword-headed expression. These may look like functions, but
may also look completely different. The syntax following the keyword is defined entirely by the extension. If you are an implementor, see
"PL_keyword_plugin" in perlapi for the mechanism. If you are using such a module, see the module's documentation for details of the syntax
that it defines.
Perl Functions by Category
Here are Perl's functions (including things that look like functions, like some keywords and named operators) arranged by category. Some
functions appear in more than one place.
Functions for SCALARs or strings
"chomp", "chop", "chr", "crypt", "fc", "hex", "index", "lc", "lcfirst", "length", "oct", "ord", "pack", "q//", "qq//", "reverse",
"rindex", "sprintf", "substr", "tr///", "uc", "ucfirst", "y///"
"fc" is available only if the "fc" feature is enabled or if it is prefixed with "CORE::". The "fc" feature is enabled automatically
with a "use v5.16" (or higher) declaration in the current scope.
Regular expressions and pattern matching
"m//", "pos", "qr//", "quotemeta", "s///", "split", "study"
Numeric functions
"abs", "atan2", "cos", "exp", "hex", "int", "log", "oct", "rand", "sin", "sqrt", "srand"
Functions for real @ARRAYs
"each", "keys", "pop", "push", "shift", "splice", "unshift", "values"
Functions for list data
"grep", "join", "map", "qw//", "reverse", "sort", "unpack"
Functions for real %HASHes
"delete", "each", "exists", "keys", "values"
Input and output functions
"binmode", "close", "closedir", "dbmclose", "dbmopen", "die", "eof", "fileno", "flock", "format", "getc", "print", "printf", "read",
"readdir", "readline" "rewinddir", "say", "seek", "seekdir", "select", "syscall", "sysread", "sysseek", "syswrite", "tell", "telldir",
"truncate", "warn", "write"
"say" is available only if the "say" feature is enabled or if it is prefixed with "CORE::". The "say" feature is enabled automatically
with a "use v5.10" (or higher) declaration in the current scope.
Functions for fixed-length data or records
"pack", "read", "syscall", "sysread", "sysseek", "syswrite", "unpack", "vec"
Functions for filehandles, files, or directories
"-X", "chdir", "chmod", "chown", "chroot", "fcntl", "glob", "ioctl", "link", "lstat", "mkdir", "open", "opendir", "readlink", "rename",
"rmdir", "stat", "symlink", "sysopen", "umask", "unlink", "utime"
Keywords related to the control flow of your Perl program
"break", "caller", "continue", "die", "do", "dump", "eval", "evalbytes" "exit", "__FILE__", "goto", "last", "__LINE__", "next",
"__PACKAGE__", "redo", "return", "sub", "__SUB__", "wantarray"
"break" is available only if you enable the experimental "switch" feature or use the "CORE::" prefix. The "switch" feature also enables
the "default", "given" and "when" statements, which are documented in "Switch Statements" in perlsyn. The "switch" feature is enabled
automatically with a "use v5.10" (or higher) declaration in the current scope. In Perl v5.14 and earlier, "continue" required the
"switch" feature, like the other keywords.
"evalbytes" is only available with with the "evalbytes" feature (see feature) or if prefixed with "CORE::". "__SUB__" is only
available with with the "current_sub" feature or if prefixed with "CORE::". Both the "evalbytes" and "current_sub" features are enabled
automatically with a "use v5.16" (or higher) declaration in the current scope.
Keywords related to scoping
"caller", "import", "local", "my", "our", "package", "state", "use"
"state" is available only if the "state" feature is enabled or if it is prefixed with "CORE::". The "state" feature is enabled
automatically with a "use v5.10" (or higher) declaration in the current scope.
Miscellaneous functions
"defined", "formline", "lock", "prototype", "reset", "scalar", "undef"
Functions for processes and process groups
"alarm", "exec", "fork", "getpgrp", "getppid", "getpriority", "kill", "pipe", "qx//", "readpipe", "setpgrp", "setpriority", "sleep",
"system", "times", "wait", "waitpid"
Keywords related to Perl modules
"do", "import", "no", "package", "require", "use"
Keywords related to classes and object-orientation
"bless", "dbmclose", "dbmopen", "package", "ref", "tie", "tied", "untie", "use"
Low-level socket functions
"accept", "bind", "connect", "getpeername", "getsockname", "getsockopt", "listen", "recv", "send", "setsockopt", "shutdown", "socket",
"socketpair"
System V interprocess communication functions
"msgctl", "msgget", "msgrcv", "msgsnd", "semctl", "semget", "semop", "shmctl", "shmget", "shmread", "shmwrite"
Fetching user and group info
"endgrent", "endhostent", "endnetent", "endpwent", "getgrent", "getgrgid", "getgrnam", "getlogin", "getpwent", "getpwnam", "getpwuid",
"setgrent", "setpwent"
Fetching network info
"endprotoent", "endservent", "gethostbyaddr", "gethostbyname", "gethostent", "getnetbyaddr", "getnetbyname", "getnetent",
"getprotobyname", "getprotobynumber", "getprotoent", "getservbyname", "getservbyport", "getservent", "sethostent", "setnetent",
"setprotoent", "setservent"
Time-related functions
"gmtime", "localtime", "time", "times"
Non-function keywords
"and", "AUTOLOAD", "BEGIN", "CHECK", "cmp", "CORE", "__DATA__", "default", "DESTROY", "else", "elseif", "elsif", "END", "__END__",
"eq", "for", "foreach", "ge", "given", "gt", "if", "INIT", "le", "lt", "ne", "not", "or", "UNITCHECK", "unless", "until", "when",
"while", "x", "xor"
Portability
Perl was born in Unix and can therefore access all common Unix system calls. In non-Unix environments, the functionality of some Unix
system calls may not be available or details of the available functionality may differ slightly. The Perl functions affected by this are:
"-X", "binmode", "chmod", "chown", "chroot", "crypt", "dbmclose", "dbmopen", "dump", "endgrent", "endhostent", "endnetent", "endprotoent",
"endpwent", "endservent", "exec", "fcntl", "flock", "fork", "getgrent", "getgrgid", "gethostbyname", "gethostent", "getlogin",
"getnetbyaddr", "getnetbyname", "getnetent", "getppid", "getpgrp", "getpriority", "getprotobynumber", "getprotoent", "getpwent",
"getpwnam", "getpwuid", "getservbyport", "getservent", "getsockopt", "glob", "ioctl", "kill", "link", "lstat", "msgctl", "msgget",
"msgrcv", "msgsnd", "open", "pipe", "readlink", "rename", "select", "semctl", "semget", "semop", "setgrent", "sethostent", "setnetent",
"setpgrp", "setpriority", "setprotoent", "setpwent", "setservent", "setsockopt", "shmctl", "shmget", "shmread", "shmwrite", "socket",
"socketpair", "stat", "symlink", "syscall", "sysopen", "system", "times", "truncate", "umask", "unlink", "utime", "wait", "waitpid"
For more information about the portability of these functions, see perlport and other available platform-specific documentation.
Alphabetical Listing of Perl Functions
-X FILEHANDLE
-X EXPR
-X DIRHANDLE
-X A file test, where X is one of the letters listed below. This unary operator takes one argument, either a filename, a filehandle, or a
dirhandle, and tests the associated file to see if something is true about it. If the argument is omitted, tests $_, except for "-t",
which tests STDIN. Unless otherwise documented, it returns 1 for true and '' for false, or the undefined value if the file doesn't
exist. Despite the funny names, precedence is the same as any other named unary operator. The operator may be any of:
-r File is readable by effective uid/gid.
-w File is writable by effective uid/gid.
-x File is executable by effective uid/gid.
-o File is owned by effective uid.
-R File is readable by real uid/gid.
-W File is writable by real uid/gid.
-X File is executable by real uid/gid.
-O File is owned by real uid.
-e File exists.
-z File has zero size (is empty).
-s File has nonzero size (returns size in bytes).
-f File is a plain file.
-d File is a directory.
-l File is a symbolic link.
-p File is a named pipe (FIFO), or Filehandle is a pipe.
-S File is a socket.
-b File is a block special file.
-c File is a character special file.
-t Filehandle is opened to a tty.
-u File has setuid bit set.
-g File has setgid bit set.
-k File has sticky bit set.
-T File is an ASCII text file (heuristic guess).
-B File is a "binary" file (opposite of -T).
-M Script start time minus file modification time, in days.
-A Same for access time.
-C Same for inode change time (Unix, may differ for other platforms)
Example:
while (<>) {
chomp;
next unless -f $_; # ignore specials
#...
}
Note that "-s/a/b/" does not do a negated substitution. Saying "-exp($foo)" still works as expected, however: only single letters
following a minus are interpreted as file tests.
These operators are exempt from the "looks like a function rule" described above. That is, an opening parenthesis after the operator
does not affect how much of the following code constitutes the argument. Put the opening parentheses before the operator to separate
it from code that follows (this applies only to operators with higher precedence than unary operators, of course):
-s($file) + 1024 # probably wrong; same as -s($file + 1024)
(-s $file) + 1024 # correct
The interpretation of the file permission operators "-r", "-R", "-w", "-W", "-x", and "-X" is by default based solely on the mode of
the file and the uids and gids of the user. There may be other reasons you can't actually read, write, or execute the file: for
example network filesystem access controls, ACLs (access control lists), read-only filesystems, and unrecognized executable formats.
Note that the use of these six specific operators to verify if some operation is possible is usually a mistake, because it may be open
to race conditions.
Also note that, for the superuser on the local filesystems, the "-r", "-R", "-w", and "-W" tests always return 1, and "-x" and "-X"
return 1 if any execute bit is set in the mode. Scripts run by the superuser may thus need to do a stat() to determine the actual mode
of the file, or temporarily set their effective uid to something else.
If you are using ACLs, there is a pragma called "filetest" that may produce more accurate results than the bare stat() mode bits. When
under "use filetest 'access'" the above-mentioned filetests test whether the permission can(not) be granted using the access(2) family
of system calls. Also note that the "-x" and "-X" may under this pragma return true even if there are no execute permission bits set
(nor any extra execute permission ACLs). This strangeness is due to the underlying system calls' definitions. Note also that, due to
the implementation of "use filetest 'access'", the "_" special filehandle won't cache the results of the file tests when this pragma is
in effect. Read the documentation for the "filetest" pragma for more information.
The "-T" and "-B" switches work as follows. The first block or so of the file is examined for odd characters such as strange control
codes or characters with the high bit set. If too many strange characters (>30%) are found, it's a "-B" file; otherwise it's a "-T"
file. Also, any file containing a zero byte in the first block is considered a binary file. If "-T" or "-B" is used on a filehandle,
the current IO buffer is examined rather than the first block. Both "-T" and "-B" return true on an empty file, or a file at EOF when
testing a filehandle. Because you have to read a file to do the "-T" test, on most occasions you want to use a "-f" against the file
first, as in "next unless -f $file && -T $file".
If any of the file tests (or either the "stat" or "lstat" operator) is given the special filehandle consisting of a solitary underline,
then the stat structure of the previous file test (or stat operator) is used, saving a system call. (This doesn't work with "-t", and
you need to remember that lstat() and "-l" leave values in the stat structure for the symbolic link, not the real file.) (Also, if the
stat buffer was filled by an "lstat" call, "-T" and "-B" will reset it with the results of "stat _"). Example:
print "Can do.
" if -r $a || -w _ || -x _;
stat($filename);
print "Readable
" if -r _;
print "Writable
" if -w _;
print "Executable
" if -x _;
print "Setuid
" if -u _;
print "Setgid
" if -g _;
print "Sticky
" if -k _;
print "Text
" if -T _;
print "Binary
" if -B _;
As of Perl 5.9.1, as a form of purely syntactic sugar, you can stack file test operators, in a way that "-f -w -x $file" is equivalent
to "-x $file && -w _ && -f _". (This is only fancy fancy: if you use the return value of "-f $file" as an argument to another filetest
operator, no special magic will happen.)
Portability issues: "-X" in perlport.
To avoid confusing would-be users of your code with mysterious syntax errors, put something like this at the top of your script:
use 5.010; # so filetest ops can stack
abs VALUE
abs Returns the absolute value of its argument. If VALUE is omitted, uses $_.
accept NEWSOCKET,GENERICSOCKET
Accepts an incoming socket connect, just as accept(2) does. Returns the packed address if it succeeded, false otherwise. See the
example in "Sockets: Client/Server Communication" in perlipc.
On systems that support a close-on-exec flag on files, the flag will be set for the newly opened file descriptor, as determined by the
value of $^F. See "$^F" in perlvar.
alarm SECONDS
alarm
Arranges to have a SIGALRM delivered to this process after the specified number of wallclock seconds has elapsed. If SECONDS is not
specified, the value stored in $_ is used. (On some machines, unfortunately, the elapsed time may be up to one second less or more
than you specified because of how seconds are counted, and process scheduling may delay the delivery of the signal even further.)
Only one timer may be counting at once. Each call disables the previous timer, and an argument of 0 may be supplied to cancel the
previous timer without starting a new one. The returned value is the amount of time remaining on the previous timer.
For delays of finer granularity than one second, the Time::HiRes module (from CPAN, and starting from Perl 5.8 part of the standard
distribution) provides ualarm(). You may also use Perl's four-argument version of select() leaving the first three arguments
undefined, or you might be able to use the "syscall" interface to access setitimer(2) if your system supports it. See perlfaq8 for
details.
It is usually a mistake to intermix "alarm" and "sleep" calls, because "sleep" may be internally implemented on your system with
"alarm".
If you want to use "alarm" to time out a system call you need to use an "eval"/"die" pair. You can't rely on the alarm causing the
system call to fail with $! set to "EINTR" because Perl sets up signal handlers to restart system calls on some systems. Using
"eval"/"die" always works, modulo the caveats given in "Signals" in perlipc.
eval {
local $SIG{ALRM} = sub { die "alarm
" }; # NB:
required
alarm $timeout;
$nread = sysread SOCKET, $buffer, $size;
alarm 0;
};
if ($@) {
die unless $@ eq "alarm
"; # propagate unexpected errors
# timed out
}
else {
# didn't
}
For more information see perlipc.
Portability issues: "alarm" in perlport.
atan2 Y,X
Returns the arctangent of Y/X in the range -PI to PI.
For the tangent operation, you may use the "Math::Trig::tan" function, or use the familiar relation:
sub tan { sin($_[0]) / cos($_[0]) }
The return value for "atan2(0,0)" is implementation-defined; consult your atan2(3) manpage for more information.
Portability issues: "atan2" in perlport.
bind SOCKET,NAME
Binds a network address to a socket, just as bind(2) does. Returns true if it succeeded, false otherwise. NAME should be a packed
address of the appropriate type for the socket. See the examples in "Sockets: Client/Server Communication" in perlipc.
binmode FILEHANDLE, LAYER
binmode FILEHANDLE
Arranges for FILEHANDLE to be read or written in "binary" or "text" mode on systems where the run-time libraries distinguish between
binary and text files. If FILEHANDLE is an expression, the value is taken as the name of the filehandle. Returns true on success,
otherwise it returns "undef" and sets $! (errno).
On some systems (in general, DOS- and Windows-based systems) binmode() is necessary when you're not working with a text file. For the
sake of portability it is a good idea always to use it when appropriate, and never to use it when it isn't appropriate. Also, people
can set their I/O to be by default UTF8-encoded Unicode, not bytes.
In other words: regardless of platform, use binmode() on binary data, like images, for example.
If LAYER is present it is a single string, but may contain multiple directives. The directives alter the behaviour of the filehandle.
When LAYER is present, using binmode on a text file makes sense.
If LAYER is omitted or specified as ":raw" the filehandle is made suitable for passing binary data. This includes turning off possible
CRLF translation and marking it as bytes (as opposed to Unicode characters). Note that, despite what may be implied in "Programming
Perl" (the Camel, 3rd edition) or elsewhere, ":raw" is not simply the inverse of ":crlf". Other layers that would affect the binary
nature of the stream are also disabled. See PerlIO, perlrun, and the discussion about the PERLIO environment variable.
The ":bytes", ":crlf", ":utf8", and any other directives of the form ":...", are called I/O layers. The "open" pragma can be used to
establish default I/O layers. See open.
The LAYER parameter of the binmode() function is described as "DISCIPLINE" in "Programming Perl, 3rd Edition". However, since the
publishing of this book, by many known as "Camel III", the consensus of the naming of this functionality has moved from "discipline" to
"layer". All documentation of this version of Perl therefore refers to "layers" rather than to "disciplines". Now back to the
regularly scheduled documentation...
To mark FILEHANDLE as UTF-8, use ":utf8" or ":encoding(UTF-8)". ":utf8" just marks the data as UTF-8 without further checking, while
":encoding(UTF-8)" checks the data for actually being valid UTF-8. More details can be found in PerlIO::encoding.
In general, binmode() should be called after open() but before any I/O is done on the filehandle. Calling binmode() normally flushes
any pending buffered output data (and perhaps pending input data) on the handle. An exception to this is the ":encoding" layer that
changes the default character encoding of the handle; see "open". The ":encoding" layer sometimes needs to be called in mid-stream,
and it doesn't flush the stream. The ":encoding" also implicitly pushes on top of itself the ":utf8" layer because internally Perl
operates on UTF8-encoded Unicode characters.
The operating system, device drivers, C libraries, and Perl run-time system all conspire to let the programmer treat a single character
("
") as the line terminator, irrespective of external representation. On many operating systems, the native text file representation
matches the internal representation, but on some platforms the external representation of "
" is made up of more than one character.
All variants of Unix, Mac OS (old and new), and Stream_LF files on VMS use a single character to end each line in the external
representation of text (even though that single character is CARRIAGE RETURN on old, pre-Darwin flavors of Mac OS, and is LINE FEED on
Unix and most VMS files). In other systems like OS/2, DOS, and the various flavors of MS-Windows, your program sees a "
" as a simple
"cJ", but what's stored in text files are the two characters "cMcJ". That means that if you don't use binmode() on these systems,
"cMcJ" sequences on disk will be converted to "
" on input, and any "
" in your program will be converted back to "cMcJ" on
output. This is what you want for text files, but it can be disastrous for binary files.
Another consequence of using binmode() (on some systems) is that special end-of-file markers will be seen as part of the data stream.
For systems from the Microsoft family this means that, if your binary data contain "cZ", the I/O subsystem will regard it as the end
of the file, unless you use binmode().
binmode() is important not only for readline() and print() operations, but also when using read(), seek(), sysread(), syswrite() and
tell() (see perlport for more details). See the $/ and "$" variables in perlvar for how to manually set your input and output line-
termination sequences.
Portability issues: "binmode" in perlport.
bless REF,CLASSNAME
bless REF
This function tells the thingy referenced by REF that it is now an object in the CLASSNAME package. If CLASSNAME is omitted, the
current package is used. Because a "bless" is often the last thing in a constructor, it returns the reference for convenience. Always
use the two-argument version if a derived class might inherit the function doing the blessing. Seeperlobj for more about the blessing
(and blessings) of objects.
Consider always blessing objects in CLASSNAMEs that are mixed case. Namespaces with all lowercase names are considered reserved for
Perl pragmata. Builtin types have all uppercase names. To prevent confusion, you may wish to avoid such package names as well. Make
sure that CLASSNAME is a true value.
See "Perl Modules" in perlmod.
break
Break out of a "given()" block.
This keyword is enabled by the "switch" feature: see feature for more information. You can also access it by prefixing it with
"CORE::". Alternately, include a "use v5.10" or later to the current scope.
caller EXPR
caller
Returns the context of the current subroutine call. In scalar context, returns the caller's package name if there is a caller (that
is, if we're in a subroutine or "eval" or "require") and the undefined value otherwise. In list context, returns
# 0 1 2
($package, $filename, $line) = caller;
With EXPR, it returns some extra information that the debugger uses to print a stack trace. The value of EXPR indicates how many call
frames to go back before the current one.
# 0 1 2 3 4
($package, $filename, $line, $subroutine, $hasargs,
# 5 6 7 8 9 10
$wantarray, $evaltext, $is_require, $hints, $bitmask, $hinthash)
= caller($i);
Here $subroutine may be "(eval)" if the frame is not a subroutine call, but an "eval". In such a case additional elements $evaltext
and $is_require are set: $is_require is true if the frame is created by a "require" or "use" statement, $evaltext contains the text of
the "eval EXPR" statement. In particular, for an "eval BLOCK" statement, $subroutine is "(eval)", but $evaltext is undefined. (Note
also that each "use" statement creates a "require" frame inside an "eval EXPR" frame.) $subroutine may also be "(unknown)" if this
particular subroutine happens to have been deleted from the symbol table. $hasargs is true if a new instance of @_ was set up for the
frame. $hints and $bitmask contain pragmatic hints that the caller was compiled with. The $hints and $bitmask values are subject to
change between versions of Perl, and are not meant for external use.
$hinthash is a reference to a hash containing the value of "%^H" when the caller was compiled, or "undef" if "%^H" was empty. Do not
modify the values of this hash, as they are the actual values stored in the optree.
Furthermore, when called from within the DB package in list context, and with an argument, caller returns more detailed information: it
sets the list variable @DB::args to be the arguments with which the subroutine was invoked.
Be aware that the optimizer might have optimized call frames away before "caller" had a chance to get the information. That means that
caller(N) might not return information about the call frame you expect it to, for "N > 1". In particular, @DB::args might have
information from the previous time "caller" was called.
Be aware that setting @DB::args is best effort, intended for debugging or generating backtraces, and should not be relied upon. In
particular, as @_ contains aliases to the caller's arguments, Perl does not take a copy of @_, so @DB::args will contain modifications
the subroutine makes to @_ or its contents, not the original values at call time. @DB::args, like @_, does not hold explicit
references to its elements, so under certain cases its elements may have become freed and reallocated for other variables or temporary
values. Finally, a side effect of the current implementation is that the effects of "shift @_" can normally be undone (but not "pop
@_" or other splicing, and not if a reference to @_ has been taken, and subject to the caveat about reallocated elements), so @DB::args
is actually a hybrid of the current state and initial state of @_. Buyer beware.
chdir EXPR
chdir FILEHANDLE
chdir DIRHANDLE
chdir
Changes the working directory to EXPR, if possible. If EXPR is omitted, changes to the directory specified by $ENV{HOME}, if set; if
not, changes to the directory specified by $ENV{LOGDIR}. (Under VMS, the variable $ENV{SYS$LOGIN} is also checked, and used if it is
set.) If neither is set, "chdir" does nothing. It returns true on success, false otherwise. See the example under "die".
On systems that support fchdir(2), you may pass a filehandle or directory handle as the argument. On systems that don't support
fchdir(2), passing handles raises an exception.
chmod LIST
Changes the permissions of a list of files. The first element of the list must be the numeric mode, which should probably be an octal
number, and which definitely should not be a string of octal digits: 0644 is okay, but "0644" is not. Returns the number of files
successfully changed. See also "oct" if all you have is a string.
$cnt = chmod 0755, "foo", "bar";
chmod 0755, @executables;
$mode = "0644"; chmod $mode, "foo"; # !!! sets mode to
# --w----r-T
$mode = "0644"; chmod oct($mode), "foo"; # this is better
$mode = 0644; chmod $mode, "foo"; # this is best
On systems that support fchmod(2), you may pass filehandles among the files. On systems that don't support fchmod(2), passing
filehandles raises an exception. Filehandles must be passed as globs or glob references to be recognized; barewords are considered
filenames.
open(my $fh, "<", "foo");
my $perm = (stat $fh)[2] & 07777;
chmod($perm | 0600, $fh);
You can also import the symbolic "S_I*" constants from the "Fcntl" module:
use Fcntl qw( :mode );
chmod S_IRWXU|S_IRGRP|S_IXGRP|S_IROTH|S_IXOTH, @executables;
# Identical to the chmod 0755 of the example above.
Portability issues: "chmod" in perlport.
chomp VARIABLE
chomp( LIST )
chomp
This safer version of "chop" removes any trailing string that corresponds to the current value of $/ (also known as
$INPUT_RECORD_SEPARATOR in the "English" module). It returns the total number of characters removed from all its arguments. It's
often used to remove the newline from the end of an input record when you're worried that the final record may be missing its newline.
When in paragraph mode ("$/ = """), it removes all trailing newlines from the string. When in slurp mode ("$/ = undef") or fixed-
length record mode ($/ is a reference to an integer or the like; see perlvar) chomp() won't remove anything. If VARIABLE is omitted,
it chomps $_. Example:
while (<>) {
chomp; # avoid
on last field
@array = split(/:/);
# ...
}
If VARIABLE is a hash, it chomps the hash's values, but not its keys.
You can actually chomp anything that's an lvalue, including an assignment:
chomp($cwd = `pwd`);
chomp($answer = <STDIN>);
If you chomp a list, each element is chomped, and the total number of characters removed is returned.
Note that parentheses are necessary when you're chomping anything that is not a simple variable. This is because "chomp $cwd = `pwd`;"
is interpreted as "(chomp $cwd) = `pwd`;", rather than as "chomp( $cwd = `pwd` )" which you might expect. Similarly, "chomp $a, $b" is
interpreted as "chomp($a), $b" rather than as "chomp($a, $b)".
chop VARIABLE
chop( LIST )
chop
Chops off the last character of a string and returns the character chopped. It is much more efficient than "s/.$//s" because it
neither scans nor copies the string. If VARIABLE is omitted, chops $_. If VARIABLE is a hash, it chops the hash's values, but not its
keys.
You can actually chop anything that's an lvalue, including an assignment.
If you chop a list, each element is chopped. Only the value of the last "chop" is returned.
Note that "chop" returns the last character. To return all but the last character, use "substr($string, 0, -1)".
See also "chomp".
chown LIST
Changes the owner (and group) of a list of files. The first two elements of the list must be the numeric uid and gid, in that order.
A value of -1 in either position is interpreted by most systems to leave that value unchanged. Returns the number of files
successfully changed.
$cnt = chown $uid, $gid, 'foo', 'bar';
chown $uid, $gid, @filenames;
On systems that support fchown(2), you may pass filehandles among the files. On systems that don't support fchown(2), passing
filehandles raises an exception. Filehandles must be passed as globs or glob references to be recognized; barewords are considered
filenames.
Here's an example that looks up nonnumeric uids in the passwd file:
print "User: ";
chomp($user = <STDIN>);
print "Files: ";
chomp($pattern = <STDIN>);
($login,$pass,$uid,$gid) = getpwnam($user)
or die "$user not in passwd file";
@ary = glob($pattern); # expand filenames
chown $uid, $gid, @ary;
On most systems, you are not allowed to change the ownership of the file unless you're the superuser, although you should be able to
change the group to any of your secondary groups. On insecure systems, these restrictions may be relaxed, but this is not a portable
assumption. On POSIX systems, you can detect this condition this way:
use POSIX qw(sysconf _PC_CHOWN_RESTRICTED);
$can_chown_giveaway = not sysconf(_PC_CHOWN_RESTRICTED);
Portability issues: "chmod" in perlport.
chr NUMBER
chr Returns the character represented by that NUMBER in the character set. For example, "chr(65)" is "A" in either ASCII or Unicode, and
chr(0x263a) is a Unicode smiley face.
Negative values give the Unicode replacement character (chr(0xfffd)), except under the bytes pragma, where the low eight bits of the
value (truncated to an integer) are used.
If NUMBER is omitted, uses $_.
For the reverse, use "ord".
Note that characters from 128 to 255 (inclusive) are by default internally not encoded as UTF-8 for backward compatibility reasons.
See perlunicode for more about Unicode.
chroot FILENAME
chroot
This function works like the system call by the same name: it makes the named directory the new root directory for all further
pathnames that begin with a "/" by your process and all its children. (It doesn't change your current working directory, which is
unaffected.) For security reasons, this call is restricted to the superuser. If FILENAME is omitted, does a "chroot" to $_.
Portability issues: "chroot" in perlport.
close FILEHANDLE
close
Closes the file or pipe associated with the filehandle, flushes the IO buffers, and closes the system file descriptor. Returns true if
those operations succeed and if no error was reported by any PerlIO layer. Closes the currently selected filehandle if the argument is
omitted.
You don't have to close FILEHANDLE if you are immediately going to do another "open" on it, because "open" closes it for you. (See
open.) However, an explicit "close" on an input file resets the line counter ($.), while the implicit close done by "open" does not.
If the filehandle came from a piped open, "close" returns false if one of the other syscalls involved fails or if its program exits
with non-zero status. If the only problem was that the program exited non-zero, $! will be set to 0. Closing a pipe also waits for
the process executing on the pipe to exit--in case you wish to look at the output of the pipe afterwards--and implicitly puts the exit
status value of that command into $? and "${^CHILD_ERROR_NATIVE}".
If there are multiple threads running, "close" on a filehandle from a piped open returns true without waiting for the child process to
terminate, if the filehandle is still open in another thread.
Closing the read end of a pipe before the process writing to it at the other end is done writing results in the writer receiving a
SIGPIPE. If the other end can't handle that, be sure to read all the data before closing the pipe.
Example:
open(OUTPUT, '|sort >foo') # pipe to sort
or die "Can't start sort: $!";
#... # print stuff to output
close OUTPUT # wait for sort to finish
or warn $! ? "Error closing sort pipe: $!"
: "Exit status $? from sort";
open(INPUT, 'foo') # get sort's results
or die "Can't open 'foo' for input: $!";
FILEHANDLE may be an expression whose value can be used as an indirect filehandle, usually the real filehandle name or an autovivified
handle.
closedir DIRHANDLE
Closes a directory opened by "opendir" and returns the success of that system call.
connect SOCKET,NAME
Attempts to connect to a remote socket, just like connect(2). Returns true if it succeeded, false otherwise. NAME should be a packed
address of the appropriate type for the socket. See the examples in "Sockets: Client/Server Communication" in perlipc.
continue BLOCK
continue
When followed by a BLOCK, "continue" is actually a flow control statement rather than a function. If there is a "continue" BLOCK
attached to a BLOCK (typically in a "while" or "foreach"), it is always executed just before the conditional is about to be evaluated
again, just like the third part of a "for" loop in C. Thus it can be used to increment a loop variable, even when the loop has been
continued via the "next" statement (which is similar to the C "continue" statement).
"last", "next", or "redo" may appear within a "continue" block; "last" and "redo" behave as if they had been executed within the main
block. So will "next", but since it will execute a "continue" block, it may be more entertaining.
while (EXPR) {
### redo always comes here
do_something;
} continue {
### next always comes here
do_something_else;
# then back the top to re-check EXPR
}
### last always comes here
Omitting the "continue" section is equivalent to using an empty one, logically enough, so "next" goes directly back to check the
condition at the top of the loop.
When there is no BLOCK, "continue" is a function that falls through the current "when" or "default" block instead of iterating a
dynamically enclosing "foreach" or exiting a lexically enclosing "given". In Perl 5.14 and earlier, this form of "continue" was only
available when the "switch" feature was enabled. See feature and "Switch Statements" in perlsyn for more information.
cos EXPR
cos Returns the cosine of EXPR (expressed in radians). If EXPR is omitted, takes the cosine of $_.
For the inverse cosine operation, you may use the "Math::Trig::acos()" function, or use this relation:
sub acos { atan2( sqrt(1 - $_[0] * $_[0]), $_[0] ) }
crypt PLAINTEXT,SALT
Creates a digest string exactly like the crypt(3) function in the C library (assuming that you actually have a version there that has
not been extirpated as a potential munition).
crypt() is a one-way hash function. The PLAINTEXT and SALT are turned into a short string, called a digest, which is returned. The
same PLAINTEXT and SALT will always return the same string, but there is no (known) way to get the original PLAINTEXT from the hash.
Small changes in the PLAINTEXT or SALT will result in large changes in the digest.
There is no decrypt function. This function isn't all that useful for cryptography (for that, look for Crypt modules on your nearby
CPAN mirror) and the name "crypt" is a bit of a misnomer. Instead it is primarily used to check if two pieces of text are the same
without having to transmit or store the text itself. An example is checking if a correct password is given. The digest of the
password is stored, not the password itself. The user types in a password that is crypt()'d with the same salt as the stored digest.
If the two digests match, the password is correct.
When verifying an existing digest string you should use the digest as the salt (like "crypt($plain, $digest) eq $digest"). The SALT
used to create the digest is visible as part of the digest. This ensures crypt() will hash the new string with the same salt as the
digest. This allows your code to work with the standard crypt and with more exotic implementations. In other words, assume nothing
about the returned string itself nor about how many bytes of SALT may matter.
Traditionally the result is a string of 13 bytes: two first bytes of the salt, followed by 11 bytes from the set "[./0-9A-Za-z]", and
only the first eight bytes of PLAINTEXT mattered. But alternative hashing schemes (like MD5), higher level security schemes (like C2),
and implementations on non-Unix platforms may produce different strings.
When choosing a new salt create a random two character string whose characters come from the set "[./0-9A-Za-z]" (like "join '', ('.',
'/', 0..9, 'A'..'Z', 'a'..'z')[rand 64, rand 64]"). This set of characters is just a recommendation; the characters allowed in the
salt depend solely on your system's crypt library, and Perl can't restrict what salts "crypt()" accepts.
Here's an example that makes sure that whoever runs this program knows their password:
$pwd = (getpwuid($<))[1];
system "stty -echo";
print "Password: ";
chomp($word = <STDIN>);
print "
";
system "stty echo";
if (crypt($word, $pwd) ne $pwd) {
die "Sorry...
";
} else {
print "ok
";
}
Of course, typing in your own password to whoever asks you for it is unwise.
The crypt function is unsuitable for hashing large quantities of data, not least of all because you can't get the information back.
Look at the Digest module for more robust algorithms.
If using crypt() on a Unicode string (which potentially has characters with codepoints above 255), Perl tries to make sense of the
situation by trying to downgrade (a copy of) the string back to an eight-bit byte string before calling crypt() (on that copy). If
that works, good. If not, crypt() dies with "Wide character in crypt".
Portability issues: "crypt" in perlport.
dbmclose HASH
[This function has been largely superseded by the "untie" function.]
Breaks the binding between a DBM file and a hash.
Portability issues: "dbmclose" in perlport.
dbmopen HASH,DBNAME,MASK
[This function has been largely superseded by the tie function.]
This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a hash. HASH is the name of the hash. (Unlike normal "open",
the first argument is not a filehandle, even though it looks like one). DBNAME is the name of the database (without the .dir or .pag
extension if any). If the database does not exist, it is created with protection specified by MASK (as modified by the "umask"). To
prevent creation of the database if it doesn't exist, you may specify a MODE of 0, and the function will return a false value if it
can't find an existing database. If your system supports only the older DBM functions, you may make only one "dbmopen" call in your
program. In older versions of Perl, if your system had neither DBM nor ndbm, calling "dbmopen" produced a fatal error; it now falls
back to sdbm(3).
If you don't have write access to the DBM file, you can only read hash variables, not set them. If you want to test whether you can
write, either use file tests or try setting a dummy hash entry inside an "eval" to trap the error.
Note that functions such as "keys" and "values" may return huge lists when used on large DBM files. You may prefer to use the "each"
function to iterate over large DBM files. Example:
# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
print $key, ' = ', unpack('L',$val), "
";
}
dbmclose(%HIST);
See also AnyDBM_File for a more general description of the pros and cons of the various dbm approaches, as well as DB_File for a
particularly rich implementation.
You can control which DBM library you use by loading that library before you call dbmopen():
use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
or die "Can't open netscape history file: $!";
Portability issues: "dbmopen" in perlport.
defined EXPR
defined
Returns a Boolean value telling whether EXPR has a value other than the undefined value "undef". If EXPR is not present, $_ is
checked.
Many operations return "undef" to indicate failure, end of file, system error, uninitialized variable, and other exceptional
conditions. This function allows you to distinguish "undef" from other values. (A simple Boolean test will not distinguish among
"undef", zero, the empty string, and "0", which are all equally false.) Note that since "undef" is a valid scalar, its presence
doesn't necessarily indicate an exceptional condition: "pop" returns "undef" when its argument is an empty array, or when the element
to return happens to be "undef".
You may also use "defined(&func)" to check whether subroutine &func has ever been defined. The return value is unaffected by any
forward declarations of &func. A subroutine that is not defined may still be callable: its package may have an "AUTOLOAD" method that
makes it spring into existence the first time that it is called; see perlsub.
Use of "defined" on aggregates (hashes and arrays) is deprecated. It used to report whether memory for that aggregate had ever been
allocated. This behavior may disappear in future versions of Perl. You should instead use a simple test for size:
if (@an_array) { print "has array elements
" }
if (%a_hash) { print "has hash members
" }
When used on a hash element, it tells you whether the value is defined, not whether the key exists in the hash. Use "exists" for the
latter purpose.
Examples:
print if defined $switch{D};
print "$val
" while defined($val = pop(@ary));
die "Can't readlink $sym: $!"
unless defined($value = readlink $sym);
sub foo { defined &$bar ? &$bar(@_) : die "No bar"; }
$debugging = 0 unless defined $debugging;
Note: Many folks tend to overuse "defined" and are then surprised to discover that the number 0 and "" (the zero-length string) are,
in fact, defined values. For example, if you say
"ab" =~ /a(.*)b/;
The pattern match succeeds and $1 is defined, although it matched "nothing". It didn't really fail to match anything. Rather, it
matched something that happened to be zero characters long. This is all very above-board and honest. When a function returns an
undefined value, it's an admission that it couldn't give you an honest answer. So you should use "defined" only when questioning the
integrity of what you're trying to do. At other times, a simple comparison to 0 or "" is what you want.
See also "undef", "exists", "ref".
delete EXPR
Given an expression that specifies an element or slice of a hash, "delete" deletes the specified elements from that hash so that
exists() on that element no longer returns true. Setting a hash element to the undefined value does not remove its key, but deleting
it does; see "exists".
In list context, returns the value or values deleted, or the last such element in scalar context. The return list's length always
matches that of the argument list: deleting non-existent elements returns the undefined value in their corresponding positions.
delete() may also be used on arrays and array slices, but its behavior is less straightforward. Although exists() will return false
for deleted entries, deleting array elements never changes indices of existing values; use shift() or splice() for that. However, if
all deleted elements fall at the end of an array, the array's size shrinks to the position of the highest element that still tests true
for exists(), or to 0 if none do.
WARNING: Calling delete on array values is deprecated and likely to be removed in a future version of Perl.
Deleting from %ENV modifies the environment. Deleting from a hash tied to a DBM file deletes the entry from the DBM file. Deleting
from a "tied" hash or array may not necessarily return anything; it depends on the implementation of the "tied" package's DELETE
method, which may do whatever it pleases.
The "delete local EXPR" construct localizes the deletion to the current block at run time. Until the block exits, elements locally
deleted temporarily no longer exist. See "Localized deletion of elements of composite types" in perlsub.
%hash = (foo => 11, bar => 22, baz => 33);
$scalar = delete $hash{foo}; # $scalar is 11
$scalar = delete @hash{qw(foo bar)}; # $scalar is 22
@array = delete @hash{qw(foo bar baz)}; # @array is (undef,undef,33)
The following (inefficiently) deletes all the values of %HASH and @ARRAY:
foreach $key (keys %HASH) {
delete $HASH{$key};
}
foreach $index (0 .. $#ARRAY) {
delete $ARRAY[$index];
}
And so do these:
delete @HASH{keys %HASH};
delete @ARRAY[0 .. $#ARRAY];
But both are slower than assigning the empty list or undefining %HASH or @ARRAY, which is the customary way to empty out an aggregate:
%HASH = (); # completely empty %HASH
undef %HASH; # forget %HASH ever existed
@ARRAY = (); # completely empty @ARRAY
undef @ARRAY; # forget @ARRAY ever existed
The EXPR can be arbitrarily complicated provided its final operation is an element or slice of an aggregate:
delete $ref->[$x][$y]{$key};
delete @{$ref->[$x][$y]}{$key1, $key2, @morekeys};
delete $ref->[$x][$y][$index];
delete @{$ref->[$x][$y]}[$index1, $index2, @moreindices];
die LIST
"die" raises an exception. Inside an "eval" the error message is stuffed into $@ and the "eval" is terminated with the undefined
value. If the exception is outside of all enclosing "eval"s, then the uncaught exception prints LIST to "STDERR" and exits with a non-
zero value. If you need to exit the process with a specific exit code, see "exit".
Equivalent examples:
die "Can't cd to spool: $!
" unless chdir '/usr/spool/news';
chdir '/usr/spool/news' or die "Can't cd to spool: $!
"
If the last element of LIST does not end in a newline, the current script line number and input line number (if any) are also printed,
and a newline is supplied. Note that the "input line number" (also known as "chunk") is subject to whatever notion of "line" happens
to be currently in effect, and is also available as the special variable $.. See "$/" in perlvar and "$." in perlvar.
Hint: sometimes appending ", stopped" to your message will cause it to make better sense when the string "at foo line 123" is appended.
Suppose you are running script "canasta".
die "/etc/games is no good";
die "/etc/games is no good, stopped";
produce, respectively
/etc/games is no good at canasta line 123.
/etc/games is no good, stopped at canasta line 123.
If the output is empty and $@ already contains a value (typically from a previous eval) that value is reused after appending
" ...propagated". This is useful for propagating exceptions:
eval { ... };
die unless $@ =~ /Expected exception/;
If the output is empty and $@ contains an object reference that has a "PROPAGATE" method, that method will be called with additional
file and line number parameters. The return value replaces the value in $@; i.e., as if "$@ = eval { $@->PROPAGATE(__FILE__,
__LINE__) };" were called.
If $@ is empty then the string "Died" is used.
If an uncaught exception results in interpreter exit, the exit code is determined from the values of $! and $? with this pseudocode:
exit $! if $!; # errno
exit $? >> 8 if $? >> 8; # child exit status
exit 255; # last resort
The intent is to squeeze as much possible information about the likely cause into the limited space of the system exit code. However,
as $! is the value of C's "errno", which can be set by any system call, this means that the value of the exit code used by "die" can be
non-predictable, so should not be relied upon, other than to be non-zero.
You can also call "die" with a reference argument, and if this is trapped within an "eval", $@ contains that reference. This permits
more elaborate exception handling using objects that maintain arbitrary state about the exception. Such a scheme is sometimes
preferable to matching particular string values of $@ with regular expressions. Because $@ is a global variable and "eval" may be used
within object implementations, be careful that analyzing the error object doesn't replace the reference in the global variable. It's
easiest to make a local copy of the reference before any manipulations. Here's an example:
use Scalar::Util "blessed";
eval { ... ; die Some::Module::Exception->new( FOO => "bar" ) };
if (my $ev_err = $@) {
if (blessed($ev_err) && $ev_err->isa("Some::Module::Exception")) {
# handle Some::Module::Exception
}
else {
# handle all other possible exceptions
}
}
Because Perl stringifies uncaught exception messages before display, you'll probably want to overload stringification operations on
exception objects. See overload for details about that.
You can arrange for a callback to be run just before the "die" does its deed, by setting the $SIG{__DIE__} hook. The associated
handler is called with the error text and can change the error message, if it sees fit, by calling "die" again. See "%SIG" in perlvar
for details on setting %SIG entries, and "eval BLOCK" for some examples. Although this feature was to be run only right before your
program was to exit, this is not currently so: the $SIG{__DIE__} hook is currently called even inside eval()ed blocks/strings! If one
wants the hook to do nothing in such situations, put
die @_ if $^S;
as the first line of the handler (see "$^S" in perlvar). Because this promotes strange action at a distance, this counterintuitive
behavior may be fixed in a future release.
See also exit(), warn(), and the Carp module.
do BLOCK
Not really a function. Returns the value of the last command in the sequence of commands indicated by BLOCK. When modified by the
"while" or "until" loop modifier, executes the BLOCK once before testing the loop condition. (On other statements the loop modifiers
test the conditional first.)
"do BLOCK" does not count as a loop, so the loop control statements "next", "last", or "redo" cannot be used to leave or restart the
block. See perlsyn for alternative strategies.
do SUBROUTINE(LIST)
This form of subroutine call is deprecated. SUBROUTINE can be a bareword, a scalar variable or a subroutine beginning with "&".
do EXPR
Uses the value of EXPR as a filename and executes the contents of the file as a Perl script.
do 'stat.pl';
is just like
eval `cat stat.pl`;
except that it's more efficient and concise, keeps track of the current filename for error messages, searches the @INC directories, and
updates %INC if the file is found. See "@INC" in perlvar and "%INC" in perlvar for these variables. It also differs in that code
evaluated with "do FILENAME" cannot see lexicals in the enclosing scope; "eval STRING" does. It's the same, however, in that it does
reparse the file every time you call it, so you probably don't want to do this inside a loop.
If "do" can read the file but cannot compile it, it returns "undef" and sets an error message in $@. If "do" cannot read the file, it
returns undef and sets $! to the error. Always check $@ first, as compilation could fail in a way that also sets $!. If the file is
successfully compiled, "do" returns the value of the last expression evaluated.
Inclusion of library modules is better done with the "use" and "require" operators, which also do automatic error checking and raise an
exception if there's a problem.
You might like to use "do" to read in a program configuration file. Manual error checking can be done this way:
# read in config files: system first, then user
for $file ("/share/prog/defaults.rc",
"$ENV{HOME}/.someprogrc")
{
unless ($return = do $file) {
warn "couldn't parse $file: $@" if $@;
warn "couldn't do $file: $!" unless defined $return;
warn "couldn't run $file" unless $return;
}
}
dump LABEL
dump
This function causes an immediate core dump. See also the -u command-line switch in perlrun, which does the same thing. Primarily
this is so that you can use the undump program (not supplied) to turn your core dump into an executable binary after having initialized
all your variables at the beginning of the program. When the new binary is executed it will begin by executing a "goto LABEL" (with
all the restrictions that "goto" suffers). Think of it as a goto with an intervening core dump and reincarnation. If "LABEL" is
omitted, restarts the program from the top.
WARNING: Any files opened at the time of the dump will not be open any more when the program is reincarnated, with possible resulting
confusion by Perl.
This function is now largely obsolete, mostly because it's very hard to convert a core file into an executable. That's why you should
now invoke it as "CORE::dump()", if you don't want to be warned against a possible typo.
Portability issues: "dump" in perlport.
each HASH
each ARRAY
each EXPR
When called on a hash in list context, returns a 2-element list consisting of the key and value for the next element of a hash. In
Perl 5.12 and later only, it will also return the index and value for the next element of an array so that you can iterate over it;
older Perls consider this a syntax error. When called in scalar context, returns only the key (not the value) in a hash, or the index
in an array.
Hash entries are returned in an apparently random order. The actual random order is subject to change in future versions of Perl, but
it is guaranteed to be in the same order as either the "keys" or "values" function would produce on the same (unmodified) hash. Since
Perl 5.8.2 the ordering can be different even between different runs of Perl for security reasons (see "Algorithmic Complexity Attacks"
in perlsec).
After "each" has returned all entries from the hash or array, the next call to "each" returns the empty list in list context and
"undef" in scalar context; the next call following that one restarts iteration. Each hash or array has its own internal iterator,
accessed by "each", "keys", and "values". The iterator is implicitly reset when "each" has reached the end as just described; it can
be explicitly reset by calling "keys" or "values" on the hash or array. If you add or delete a hash's elements while iterating over
it, entries may be skipped or duplicated--so don't do that. Exception: In the current implementation, it is always safe to delete the
item most recently returned by "each()", so the following code works properly:
while (($key, $value) = each %hash) {
print $key, "
";
delete $hash{$key}; # This is safe
}
This prints out your environment like the printenv(1) program, but in a different order:
while (($key,$value) = each %ENV) {
print "$key=$value
";
}
Starting with Perl 5.14, "each" can take a scalar EXPR, which must hold reference to an unblessed hash or array. The argument will be
dereferenced automatically. This aspect of "each" is considered highly experimental. The exact behaviour may change in a future
version of Perl.
while (($key,$value) = each $hashref) { ... }
To avoid confusing would-be users of your code who are running earlier versions of Perl with mysterious syntax errors, put this sort of
thing at the top of your file to signal that your code will work only on Perls of a recent vintage:
use 5.012; # so keys/values/each work on arrays
use 5.014; # so keys/values/each work on scalars (experimental)
See also "keys", "values", and "sort".
eof FILEHANDLE
eof ()
eof Returns 1 if the next read on FILEHANDLE will return end of file or if FILEHANDLE is not open. FILEHANDLE may be an expression whose
value gives the real filehandle. (Note that this function actually reads a character and then "ungetc"s it, so isn't useful in an
interactive context.) Do not read from a terminal file (or call "eof(FILEHANDLE)" on it) after end-of-file is reached. File types
such as terminals may lose the end-of-file condition if you do.
An "eof" without an argument uses the last file read. Using "eof()" with empty parentheses is different. It refers to the pseudo file
formed from the files listed on the command line and accessed via the "<>" operator. Since "<>" isn't explicitly opened, as a normal
filehandle is, an "eof()" before "<>" has been used will cause @ARGV to be examined to determine if input is available. Similarly, an
"eof()" after "<>" has returned end-of-file will assume you are processing another @ARGV list, and if you haven't set @ARGV, will read
input from "STDIN"; see "I/O Operators" in perlop.
In a "while (<>)" loop, "eof" or "eof(ARGV)" can be used to detect the end of each file, whereas "eof()" will detect the end of the
very last file only. Examples:
# reset line numbering on each input file
while (<>) {
next if /^s*#/; # skip comments
print "$. $_";
} continue {
close ARGV if eof; # Not eof()!
}
# insert dashes just before last line of last file
while (<>) {
if (eof()) { # check for end of last file
print "--------------
";
}
print;
last if eof(); # needed if we're reading from a terminal
}
Practical hint: you almost never need to use "eof" in Perl, because the input operators typically return "undef" when they run out of
data or encounter an error.
eval EXPR
eval BLOCK
eval
In the first form, the return value of EXPR is parsed and executed as if it were a little Perl program. The value of the expression
(which is itself determined within scalar context) is first parsed, and if there were no errors, executed as a block within the lexical
context of the current Perl program. This means, that in particular, any outer lexical variables are visible to it, and any package
variable settings or subroutine and format definitions remain afterwards.
Note that the value is parsed every time the "eval" executes. If EXPR is omitted, evaluates $_. This form is typically used to delay
parsing and subsequent execution of the text of EXPR until run time.
If the "unicode_eval" feature is enabled (which is the default under a "use 5.16" or higher declaration), EXPR or $_ is treated as a
string of characters, so "use utf8" declarations have no effect, and source filters are forbidden. In the absence of the
"unicode_eval" feature, the string will sometimes be treated as characters and sometimes as bytes, depending on the internal encoding,
and source filters activated within the "eval" exhibit the erratic, but historical, behaviour of affecting some outer file scope that
is still compiling. See also the "evalbytes" keyword, which always treats its input as a byte stream and works properly with source
filters, and the feature pragma.
In the second form, the code within the BLOCK is parsed only once--at the same time the code surrounding the "eval" itself was
parsed--and executed within the context of the current Perl program. This form is typically used to trap exceptions more efficiently
than the first (see below), while also providing the benefit of checking the code within BLOCK at compile time.
The final semicolon, if any, may be omitted from the value of EXPR or within the BLOCK.
In both forms, the value returned is the value of the last expression evaluated inside the mini-program; a return statement may be also
used, just as with subroutines. The expression providing the return value is evaluated in void, scalar, or list context, depending on
the context of the "eval" itself. See "wantarray" for more on how the evaluation context can be determined.
If there is a syntax error or runtime error, or a "die" statement is executed, "eval" returns "undef" in scalar context or an empty
list in list context, and $@ is set to the error message. (Prior to 5.16, a bug caused "undef" to be returned in list context for
syntax errors, but not for runtime errors.) If there was no error, $@ is set to the empty string. A control flow operator like "last"
or "goto" can bypass the setting of $@. Beware that using "eval" neither silences Perl from printing warnings to STDERR, nor does it
stuff the text of warning messages into $@. To do either of those, you have to use the $SIG{__WARN__} facility, or turn off warnings
inside the BLOCK or EXPR using "no warnings 'all'". See "warn", perlvar, warnings and perllexwarn.
Note that, because "eval" traps otherwise-fatal errors, it is useful for determining whether a particular feature (such as "socket" or
"symlink") is implemented. It is also Perl's exception-trapping mechanism, where the die operator is used to raise exceptions.
If you want to trap errors when loading an XS module, some problems with the binary interface (such as Perl version skew) may be fatal
even with "eval" unless $ENV{PERL_DL_NONLAZY} is set. See perlrun.
If the code to be executed doesn't vary, you may use the eval-BLOCK form to trap run-time errors without incurring the penalty of
recompiling each time. The error, if any, is still returned in $@. Examples:
# make divide-by-zero nonfatal
eval { $answer = $a / $b; }; warn $@ if $@;
# same thing, but less efficient
eval '$answer = $a / $b'; warn $@ if $@;
# a compile-time error
eval { $answer = }; # WRONG
# a run-time error
eval '$answer ='; # sets $@
Using the "eval{}" form as an exception trap in libraries does have some issues. Due to the current arguably broken state of "__DIE__"
hooks, you may wish not to trigger any "__DIE__" hooks that user code may have installed. You can use the "local $SIG{__DIE__}"
construct for this purpose, as this example shows:
# a private exception trap for divide-by-zero
eval { local $SIG{'__DIE__'}; $answer = $a / $b; };
warn $@ if $@;
This is especially significant, given that "__DIE__" hooks can call "die" again, which has the effect of changing their error messages:
# __DIE__ hooks may modify error messages
{
local $SIG{'__DIE__'} =
sub { (my $x = $_[0]) =~ s/foo/bar/g; die $x };
eval { die "foo lives here" };
print $@ if $@; # prints "bar lives here"
}
Because this promotes action at a distance, this counterintuitive behavior may be fixed in a future release.
With an "eval", you should be especially careful to remember what's being looked at when:
eval $x; # CASE 1
eval "$x"; # CASE 2
eval '$x'; # CASE 3
eval { $x }; # CASE 4
eval "$$x++"; # CASE 5
$$x++; # CASE 6
Cases 1 and 2 above behave identically: they run the code contained in the variable $x. (Although case 2 has misleading double quotes
making the reader wonder what else might be happening (nothing is).) Cases 3 and 4 likewise behave in the same way: they run the code
'$x', which does nothing but return the value of $x. (Case 4 is preferred for purely visual reasons, but it also has the advantage of
compiling at compile-time instead of at run-time.) Case 5 is a place where normally you would like to use double quotes, except that
in this particular situation, you can just use symbolic references instead, as in case 6.
Before Perl 5.14, the assignment to $@ occurred before restoration of localized variables, which means that for your code to run on
older versions, a temporary is required if you want to mask some but not all errors:
# alter $@ on nefarious repugnancy only
{
my $e;
{
local $@; # protect existing $@
eval { test_repugnancy() };
# $@ =~ /nefarious/ and die $@; # Perl 5.14 and higher only
$@ =~ /nefarious/ and $e = $@;
}
die $e if defined $e
}
"eval BLOCK" does not count as a loop, so the loop control statements "next", "last", or "redo" cannot be used to leave or restart the
block.
An "eval ''" executed within the "DB" package doesn't see the usual surrounding lexical scope, but rather the scope of the first non-DB
piece of code that called it. You don't normally need to worry about this unless you are writing a Perl debugger.
evalbytes EXPR
evalbytes
This function is like "eval" with a string argument, except it always parses its argument, or $_ if EXPR is omitted, as a string of
bytes. A string containing characters whose ordinal value exceeds 255 results in an error. Source filters activated within the
evaluated code apply to the code itself.
This function is only available under the "evalbytes" feature, a "use v5.16" (or higher) declaration, or with a "CORE::" prefix. See
feature for more information.
exec LIST
exec PROGRAM LIST
The "exec" function executes a system command and never returns; use "system" instead of "exec" if you want it to return. It fails and
returns false only if the command does not exist and it is executed directly instead of via your system's command shell (see below).
Since it's a common mistake to use "exec" instead of "system", Perl warns you if "exec" is called in void context and if there is a
following statement that isn't "die", "warn", or "exit" (if "-w" is set--but you always do that, right?). If you really want to follow
an "exec" with some other statement, you can use one of these styles to avoid the warning:
exec ('foo') or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";
If there is more than one argument in LIST, or if LIST is an array with more than one value, calls execvp(3) with the arguments in
LIST. If there is only one scalar argument or an array with one element in it, the argument is checked for shell metacharacters, and
if there are any, the entire argument is passed to the system's command shell for parsing (this is "/bin/sh -c" on Unix platforms, but
varies on other platforms). If there are no shell metacharacters in the argument, it is split into words and passed directly to
"execvp", which is more efficient. Examples:
exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";
If you don't really want to execute the first argument, but want to lie to the program you are executing about its own name, you can
specify the program you actually want to run as an "indirect object" (without a comma) in front of the LIST. (This always forces
interpretation of the LIST as a multivalued list, even if there is only a single scalar in the list.) Example:
$shell = '/bin/csh';
exec $shell '-sh'; # pretend it's a login shell
or, more directly,
exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system shell, results are subject to its quirks and capabilities. See "`STRING`" in perlop for
details.
Using an indirect object with "exec" or "system" is also more secure. This usage (which also works fine with system()) forces
interpretation of the arguments as a multivalued list, even if the list had just one argument. That way you're safe from the shell
expanding wildcards or splitting up words with whitespace in them.
@args = ( "echo surprise" );
exec @args; # subject to shell escapes
# if @args == 1
exec { $args[0] } @args; # safe even with one-arg list
The first version, the one without the indirect object, ran the echo program, passing it "surprise" an argument. The second version
didn't; it tried to run a program named "echo surprise", didn't find it, and set $? to a non-zero value indicating failure.
Beginning with v5.6.0, Perl attempts to flush all files opened for output before the exec, but this may not be supported on some
platforms (see perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the "autoflush()" method of "IO::Handle"
on any open handles to avoid lost output.
Note that "exec" will not call your "END" blocks, nor will it invoke "DESTROY" methods on your objects.
Portability issues: "exec" in perlport.
exists EXPR
Given an expression that specifies an element of a hash, returns true if the specified element in the hash has ever been initialized,
even if the corresponding value is undefined.
print "Exists
" if exists $hash{$key};
print "Defined
" if defined $hash{$key};
print "True
" if $hash{$key};
exists may also be called on array elements, but its behavior is much less obvious and is strongly tied to the use of "delete" on
arrays. Be aware that calling exists on array values is deprecated and likely to be removed in a future version of Perl.
print "Exists
" if exists $array[$index];
print "Defined
" if defined $array[$index];
print "True
" if $array[$index];
A hash or array element can be true only if it's defined and defined only if it exists, but the reverse doesn't necessarily hold true.
Given an expression that specifies the name of a subroutine, returns true if the specified subroutine has ever been declared, even if
it is undefined. Mentioning a subroutine name for exists or defined does not count as declaring it. Note that a subroutine that does
not exist may still be callable: its package may have an "AUTOLOAD" method that makes it spring into existence the first time that it
is called; see perlsub.
print "Exists
" if exists &subroutine;
print "Defined
" if defined &subroutine;
Note that the EXPR can be arbitrarily complicated as long as the final operation is a hash or array key lookup or subroutine name:
if (exists $ref->{A}->{B}->{$key}) { }
if (exists $hash{A}{B}{$key}) { }
if (exists $ref->{A}->{B}->[$ix]) { }
if (exists $hash{A}{B}[$ix]) { }
if (exists &{$ref->{A}{B}{$key}}) { }
Although the most deeply nested array or hash element will not spring into existence just because its existence was tested, any
intervening ones will. Thus "$ref->{"A"}" and "$ref->{"A"}->{"B"}" will spring into existence due to the existence test for the $key
element above. This happens anywhere the arrow operator is used, including even here:
undef $ref;
if (exists $ref->{"Some key"}) { }
print $ref; # prints HASH(0x80d3d5c)
This surprising autovivification in what does not at first--or even second--glance appear to be an lvalue context may be fixed in a
future release.
Use of a subroutine call, rather than a subroutine name, as an argument to exists() is an error.
exists ⊂ # OK
exists &sub(); # Error
exit EXPR
exit
Evaluates EXPR and exits immediately with that value. Example:
$ans = <STDIN>;
exit 0 if $ans =~ /^[Xx]/;
See also "die". If EXPR is omitted, exits with 0 status. The only universally recognized values for EXPR are 0 for success and 1 for
error; other values are subject to interpretation depending on the environment in which the Perl program is running. For example,
exiting 69 (EX_UNAVAILABLE) from a sendmail incoming-mail filter will cause the mailer to return the item undelivered, but that's not
true everywhere.
Don't use "exit" to abort a subroutine if there's any chance that someone might want to trap whatever error happened. Use "die"
instead, which can be trapped by an "eval".
The exit() function does not always exit immediately. It calls any defined "END" routines first, but these "END" routines may not
themselves abort the exit. Likewise any object destructors that need to be called are called before the real exit. "END" routines and
destructors can change the exit status by modifying $?. If this is a problem, you can call "POSIX::_exit($status)" to avoid END and
destructor processing. See perlmod for details.
Portability issues: "exit" in perlport.
exp EXPR
exp Returns e (the natural logarithm base) to the power of EXPR. If EXPR is omitted, gives "exp($_)".
fc EXPR
fc Returns the casefolded version of EXPR. This is the internal function implementing the "F" escape in double-quoted strings.
Casefolding is the process of mapping strings to a form where case differences are erased; comparing two strings in their casefolded
form is effectively a way of asking if two strings are equal, regardless of case.
Roughly, if you ever found yourself writing this
lc($this) eq lc($that) # Wrong!
# or
uc($this) eq uc($that) # Also wrong!
# or
$this =~ /Q$that/i # Right!
Now you can write
fc($this) eq fc($that)
And get the correct results.
Perl only implements the full form of casefolding. For further information on casefolding, refer to the Unicode Standard, specifically
sections 3.13 "Default Case Operations", 4.2 "Case-Normative", and 5.18 "Case Mappings", available at
<http://www.unicode.org/versions/latest/>, as well as the Case Charts available at <http://www.unicode.org/charts/case/>.
If EXPR is omitted, uses $_.
This function behaves the same way under various pragma, such as in a locale, as "lc" does.
While the Unicode Standard defines two additional forms of casefolding, one for Turkic languages and one that never maps one character
into multiple characters, these are not provided by the Perl core; However, the CPAN module "Unicode::Casing" may be used to provide an
implementation.
This keyword is available only when the "fc" feature is enabled, or when prefixed with "CORE::"; See feature. Alternately, include a
"use v5.16" or later to the current scope.
fcntl FILEHANDLE,FUNCTION,SCALAR
Implements the fcntl(2) function. You'll probably have to say
use Fcntl;
first to get the correct constant definitions. Argument processing and value returned work just like "ioctl" below. For example:
use Fcntl;
fcntl($filehandle, F_GETFL, $packed_return_buffer)
or die "can't fcntl F_GETFL: $!";
You don't have to check for "defined" on the return from "fcntl". Like "ioctl", it maps a 0 return from the system call into "0 but
true" in Perl. This string is true in boolean context and 0 in numeric context. It is also exempt from the normal -w warnings on
improper numeric conversions.
Note that "fcntl" raises an exception if used on a machine that doesn't implement fcntl(2). See the Fcntl module or your fcntl(2)
manpage to learn what functions are available on your system.
Here's an example of setting a filehandle named "REMOTE" to be non-blocking at the system level. You'll have to negotiate $| on your
own, though.
use Fcntl qw(F_GETFL F_SETFL O_NONBLOCK);
$flags = fcntl(REMOTE, F_GETFL, 0)
or die "Can't get flags for the socket: $!
";
$flags = fcntl(REMOTE, F_SETFL, $flags | O_NONBLOCK)
or die "Can't set flags for the socket: $!
";
Portability issues: "fcntl" in perlport.
__FILE__
A special token that returns the name of the file in which it occurs.
fileno FILEHANDLE
Returns the file descriptor for a filehandle, or undefined if the filehandle is not open. If there is no real file descriptor at the
OS level, as can happen with filehandles connected to memory objects via "open" with a reference for the third argument, -1 is
returned.
This is mainly useful for constructing bitmaps for "select" and low-level POSIX tty-handling operations. If FILEHANDLE is an
expression, the value is taken as an indirect filehandle, generally its name.
You can use this to find out whether two handles refer to the same underlying descriptor:
if (fileno(THIS) == fileno(THAT)) {
print "THIS and THAT are dups
";
}
flock FILEHANDLE,OPERATION
Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true for success, false on failure. Produces a fatal error if used on a
machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3). "flock" is Perl's portable file-locking interface, although it
locks entire files only, not records.
Two potentially non-obvious but traditional "flock" semantics are that it waits indefinitely until the lock is granted, and that its
locks are merely advisory. Such discretionary locks are more flexible, but offer fewer guarantees. This means that programs that do
not also use "flock" may modify files locked with "flock". See perlport, your port's specific documentation, and your system-specific
local manpages for details. It's best to assume traditional behavior if you're writing portable programs. (But if you're not, you
should as always feel perfectly free to write for your own system's idiosyncrasies (sometimes called "features"). Slavish adherence to
portability concerns shouldn't get in the way of your getting your job done.)
OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with LOCK_NB. These constants are traditionally valued 1, 2, 8 and
4, but you can use the symbolic names if you import them from the Fcntl module, either individually, or as a group using the ":flock"
tag. LOCK_SH requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN releases a previously requested lock. If LOCK_NB
is bitwise-or'ed with LOCK_SH or LOCK_EX, then "flock" returns immediately rather than blocking waiting for the lock; check the return
status to see if you got it.
To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE before locking or unlocking it.
Note that the emulation built with lockf(3) doesn't provide shared locks, and it requires that FILEHANDLE be open with write intent.
These are the semantics that lockf(3) implements. Most if not all systems implement lockf(3) in terms of fcntl(2) locking, though, so
the differing semantics shouldn't bite too many people.
Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE be open with read intent to use LOCK_SH and requires that it be
open with write intent to use LOCK_EX.
Note also that some versions of "flock" cannot lock things over the network; you would need to use the more system-specific "fcntl" for
that. If you like you can force Perl to ignore your system's flock(2) function, and so provide its own fcntl(2)-based emulation, by
passing the switch "-Ud_flock" to the Configure program when you configure and build a new Perl.
Here's a mailbox appender for BSD systems.
use Fcntl qw(:flock SEEK_END); # import LOCK_* and SEEK_END constants
sub lock {
my ($fh) = @_;
flock($fh, LOCK_EX) or die "Cannot lock mailbox - $!
";
# and, in case someone appended while we were waiting...
seek($fh, 0, SEEK_END) or die "Cannot seek - $!
";
}
sub unlock {
my ($fh) = @_;
flock($fh, LOCK_UN) or die "Cannot unlock mailbox - $!
";
}
open(my $mbox, ">>", "/usr/spool/mail/$ENV{'USER'}")
or die "Can't open mailbox: $!";
lock($mbox);
print $mbox $msg,"
";
unlock($mbox);
On systems that support a real flock(2), locks are inherited across fork() calls, whereas those that must resort to the more capricious
fcntl(2) function lose their locks, making it seriously harder to write servers.
See also DB_File for other flock() examples.
Portability issues: "flock" in perlport.
fork
Does a fork(2) system call to create a new process running the same program at the same point. It returns the child pid to the parent
process, 0 to the child process, or "undef" if the fork is unsuccessful. File descriptors (and sometimes locks on those descriptors)
are shared, while everything else is copied. On most systems supporting fork(), great care has gone into making it extremely efficient
(for example, using copy-on-write technology on data pages), making it the dominant paradigm for multitasking over the last few
decades.
Beginning with v5.6.0, Perl attempts to flush all files opened for output before forking the child process, but this may not be
supported on some platforms (see perlport). To be safe, you may need to set $| ($AUTOFLUSH in English) or call the "autoflush()"
method of "IO::Handle" on any open handles to avoid duplicate output.
If you "fork" without ever waiting on your children, you will accumulate zombies. On some systems, you can avoid this by setting
$SIG{CHLD} to "IGNORE". See also perlipc for more examples of forking and reaping moribund children.
Note that if your forked child inherits system file descriptors like STDIN and STDOUT that are actually connected by a pipe or socket,
even if you exit, then the remote server (such as, say, a CGI script or a backgrounded job launched from a remote shell) won't think
you're done. You should reopen those to /dev/null if it's any issue.
On some platforms such as Windows, where the fork() system call is not available, Perl can be built to emulate fork() in the Perl
interpreter. The emulation is designed, at the level of the Perl program, to be as compatible as possible with the "Unix" fork().
However it has limitations that have to be considered in code intended to be portable. See perlfork for more details.
Portability issues: "fork" in perlport.
format
Declare a picture format for use by the "write" function. For example:
format Something =
Test: @<<<<<<<< @||||| @>>>>>
$str, $%, '$' . int($num)
.
$str = "widget";
$num = $cost/$quantity;
$~ = 'Something';
write;
See perlform for many details and examples.
formline PICTURE,LIST
This is an internal function used by "format"s, though you may call it, too. It formats (see perlform) a list of values according to
the contents of PICTURE, placing the output into the format output accumulator, $^A (or $ACCUMULATOR in English). Eventually, when a
"write" is done, the contents of $^A are written to some filehandle. You could also read $^A and then set $^A back to "". Note that a
format typically does one "formline" per line of form, but the "formline" function itself doesn't care how many newlines are embedded
in the PICTURE. This means that the "~" and "~~" tokens treat the entire PICTURE as a single line. You may therefore need to use
multiple formlines to implement a single record format, just like the "format" compiler.
Be careful if you put double quotes around the picture, because an "@" character may be taken to mean the beginning of an array name.
"formline" always returns true. See perlform for other examples.
If you are trying to use this instead of "write" to capture the output, you may find it easier to open a filehandle to a scalar ("open
$fh, ">", $output") and write to that instead.
getc FILEHANDLE
getc
Returns the next character from the input file attached to FILEHANDLE, or the undefined value at end of file or if there was an error
(in the latter case $! is set). If FILEHANDLE is omitted, reads from STDIN. This is not particularly efficient. However, it cannot
be used by itself to fetch single characters without waiting for the user to hit enter. For that, try something more like:
if ($BSD_STYLE) {
system "stty cbreak </dev/tty >/dev/tty 2>&1";
}
else {
system "stty", '-icanon', 'eol', "