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X11R7.4 - man page for perlvar (x11r4 section 1)

PERLVAR(1)			 Perl Programmers Reference Guide		       PERLVAR(1)

       perlvar - Perl predefined variables

       Predefined Names

       The following names have special meaning to Perl.  Most punctuation names have reasonable
       mnemonics, or analogs in the shells.  Nevertheless, if you wish to use long variable
       names, you need only say

	   use English;

       at the top of your program. This aliases all the short names to the long names in the cur-
       rent package. Some even have medium names, generally borrowed from awk. In general, it's
       best to use the

	   use English '-no_match_vars';

       invocation if you don't need $PREMATCH, $MATCH, or $POSTMATCH, as it avoids a certain per-
       formance hit with the use of regular expressions. See English.

       Variables that depend on the currently selected filehandle may be set by calling an appro-
       priate object method on the IO::Handle object, although this is less efficient than using
       the regular built-in variables. (Summary lines below for this contain the word HANDLE.)
       First you must say

	   use IO::Handle;

       after which you may use either

	   method HANDLE EXPR

       or more safely,


       Each method returns the old value of the IO::Handle attribute.  The methods each take an
       optional EXPR, which, if supplied, specifies the new value for the IO::Handle attribute in
       question.  If not supplied, most methods do nothing to the current value--except for aut-
       oflush(), which will assume a 1 for you, just to be different.

       Because loading in the IO::Handle class is an expensive operation, you should learn how to
       use the regular built-in variables.

       A few of these variables are considered "read-only".  This means that if you try to assign
       to this variable, either directly or indirectly through a reference, you'll raise a run-
       time exception.

       You should be very careful when modifying the default values of most special variables
       described in this document. In most cases you want to localize these variables before
       changing them, since if you don't, the change may affect other modules which rely on the
       default values of the special variables that you have changed. This is one of the correct
       ways to read the whole file at once:

	   open my $fh, "<", "foo" or die $!;
	   local $/; # enable localized slurp mode
	   my $content = <$fh>;
	   close $fh;

       But the following code is quite bad:

	   open my $fh, "<", "foo" or die $!;
	   undef $/; # enable slurp mode
	   my $content = <$fh>;
	   close $fh;

       since some other module, may want to read data from some file in the default "line mode",
       so if the code we have just presented has been executed, the global value of $/ is now
       changed for any other code running inside the same Perl interpreter.

       Usually when a variable is localized you want to make sure that this change affects the
       shortest scope possible. So unless you are already inside some short "{}" block, you
       should create one yourself. For example:

	   my $content = '';
	   open my $fh, "<", "foo" or die $!;
	       local $/;
	       $content = <$fh>;
	   close $fh;

       Here is an example of how your own code can go broken:

	   for (1..5){
	       print "$_ ";
	   sub nasty_break {
	       $_ = 5;
	       # do something with $_

       You probably expect this code to print:

	   1 2 3 4 5

       but instead you get:

	   5 5 5 5 5

       Why? Because nasty_break() modifies $_ without localizing it first. The fix is to add

	       local $_ = 5;

       It's easy to notice the problem in such a short example, but in more complicated code you
       are looking for trouble if you don't localize changes to the special variables.

       The following list is ordered by scalar variables first, then the arrays, then the hashes.

       $_      The default input and pattern-searching space.  The following pairs are equiva-

		   while (<>) {...}    # equivalent only in while!
		   while (defined($_ = <>)) {...}

		   $_ =~ /^Subject:/

		   $_ =~ tr/a-z/A-Z/


	       Here are the places where Perl will assume $_ even if you don't use it:

	       *  The following functions:

		  abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, exp, glob, hex, int,
		  lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, quotemeta, read-
		  link, readpipe, ref, require, reverse (in scalar context only), rmdir, sin,
		  split (on its second argument), sqrt, stat, study, uc, ucfirst, unlink, unpack.

	       *  All file tests ("-f", "-d") except for "-t", which defaults to STDIN.  See "-X"
		  in perlfunc

	       *  The pattern matching operations "m//", "s///" and "tr///" (aka "y///") when
		  used without an "=~" operator.

	       *  The default iterator variable in a "foreach" loop if no other variable is sup-

	       *  The implicit iterator variable in the grep() and map() functions.

	       *  The implicit variable of given().

	       *  The default place to put an input record when a "<FH>" operation's result is
		  tested by itself as the sole criterion of a "while" test.  Outside a "while"
		  test, this will not happen.

	       (Mnemonic: underline is understood in certain operations.)

       $b      Special package variables when using sort(), see "sort" in perlfunc.  Because of
	       this specialness $a and $b don't need to be declared (using use vars, or our())
	       even when using the "strict 'vars'" pragma.  Don't lexicalize them with "my $a" or
	       "my $b" if you want to be able to use them in the sort() comparison block or func-

	       Contains the subpattern from the corresponding set of capturing parentheses from
	       the last pattern match, not counting patterns matched in nested blocks that have
	       been exited already.  (Mnemonic: like \digits.)	These variables are all read-only
	       and dynamically scoped to the current BLOCK.

       $&      The string matched by the last successful pattern match (not counting any matches
	       hidden within a BLOCK or eval() enclosed by the current BLOCK).	(Mnemonic: like &
	       in some editors.)  This variable is read-only and dynamically scoped to the cur-
	       rent BLOCK.

	       The use of this variable anywhere in a program imposes a considerable performance
	       penalty on all regular expression matches.  See "BUGS".

	       See "@-" for a replacement.

       $`      The string preceding whatever was matched by the last successful pattern match
	       (not counting any matches hidden within a BLOCK or eval enclosed by the current
	       BLOCK).	(Mnemonic: "`" often precedes a quoted string.)  This variable is

	       The use of this variable anywhere in a program imposes a considerable performance
	       penalty on all regular expression matches.  See "BUGS".

	       See "@-" for a replacement.

       $'      The string following whatever was matched by the last successful pattern match
	       (not counting any matches hidden within a BLOCK or eval() enclosed by the current
	       BLOCK).	(Mnemonic: "'" often follows a quoted string.)	Example:

		   local $_ = 'abcdefghi';
		   print "$`:$&:$'\n";	       # prints abc:def:ghi

	       This variable is read-only and dynamically scoped to the current BLOCK.

	       The use of this variable anywhere in a program imposes a considerable performance
	       penalty on all regular expression matches.  See "BUGS".

	       See "@-" for a replacement.

       $+      The text matched by the last bracket of the last successful search pattern.  This
	       is useful if you don't know which one of a set of alternative patterns matched.
	       For example:

		   /Version: (.*)|Revision: (.*)/ && ($rev = $+);

	       (Mnemonic: be positive and forward looking.)  This variable is read-only and
	       dynamically scoped to the current BLOCK.

       $^N     The text matched by the used group most-recently closed (i.e. the group with the
	       rightmost closing parenthesis) of the last successful search pattern.  (Mnemonic:
	       the (possibly) Nested parenthesis that most recently closed.)

	       This is primarily used inside "(?{...})" blocks for examining text recently
	       matched. For example, to effectively capture text to a variable (in addition to
	       $1, $2, etc.), replace "(...)" with

		    (?:(...)(?{ $var = $^N }))

	       By setting and then using $var in this way relieves you from having to worry about
	       exactly which numbered set of parentheses they are.

	       This variable is dynamically scoped to the current BLOCK.

       @+      This array holds the offsets of the ends of the last successful submatches in the
	       currently active dynamic scope.	$+[0] is the offset into the string of the end of
	       the entire match.  This is the same value as what the "pos" function returns when
	       called on the variable that was matched against.  The nth element of this array
	       holds the offset of the nth submatch, so $+[1] is the offset past where $1 ends,
	       $+[2] the offset past where $2 ends, and so on.	You can use $#+ to determine how
	       many subgroups were in the last successful match.  See the examples given for the
	       "@-" variable.

       $*      Set to a non-zero integer value to do multi-line matching within a string, 0 (or
	       undefined) to tell Perl that it can assume that strings contain a single line, for
	       the purpose of optimizing pattern matches.  Pattern matches on strings containing
	       multiple newlines can produce confusing results when $* is 0 or undefined. Default
	       is undefined.  (Mnemonic: * matches multiple things.) This variable influences the
	       interpretation of only "^" and "$". A literal newline can be searched for even
	       when "$* == 0".

	       Use of $* is deprecated in modern Perl, supplanted by the "/s" and "/m" modifiers
	       on pattern matching.

	       Assigning a non-numerical value to $* triggers a warning (and makes $* act if "$*
	       == 0"), while assigning a numerical value to $* makes that an implicit "int" is
	       applied on the value.

       $.      Current line number for the last filehandle accessed.

	       Each filehandle in Perl counts the number of lines that have been read from it.
	       (Depending on the value of $/, Perl's idea of what constitutes a line may not
	       match yours.)  When a line is read from a filehandle (via readline() or "<>"), or
	       when tell() or seek() is called on it, $. becomes an alias to the line counter for
	       that filehandle.

	       You can adjust the counter by assigning to $., but this will not actually move the
	       seek pointer.  Localizing $. will not localize the filehandle's line count.
	       Instead, it will localize perl's notion of which filehandle $. is currently
	       aliased to.

	       $. is reset when the filehandle is closed, but not when an open filehandle is
	       reopened without an intervening close().  For more details, see "I/O Operators" in
	       perlop.	Because "<>" never does an explicit close, line numbers increase across
	       ARGV files (but see examples in "eof" in perlfunc).

	       You can also use "HANDLE->input_line_number(EXPR)" to access the line counter for
	       a given filehandle without having to worry about which handle you last accessed.

	       (Mnemonic: many programs use "." to mean the current line number.)

       $/      The input record separator, newline by default.	This influences Perl's idea of
	       what a "line" is.  Works like awk's RS variable, including treating empty lines as
	       a terminator if set to the null string.	(An empty line cannot contain any spaces
	       or tabs.)  You may set it to a multi-character string to match a multi-character
	       terminator, or to "undef" to read through the end of file.  Setting it to "\n\n"
	       means something slightly different than setting to "", if the file contains con-
	       secutive empty lines.  Setting to "" will treat two or more consecutive empty
	       lines as a single empty line.  Setting to "\n\n" will blindly assume that the next
	       input character belongs to the next paragraph, even if it's a newline.  (Mnemonic:
	       / delimits line boundaries when quoting poetry.)

		   local $/;	       # enable "slurp" mode
		   local $_ = <FH>;    # whole file now here
		   s/\n[ \t]+/ /g;

	       Remember: the value of $/ is a string, not a regex.  awk has to be better for
	       something. :-)

	       Setting $/ to a reference to an integer, scalar containing an integer, or scalar
	       that's convertible to an integer will attempt to read records instead of lines,
	       with the maximum record size being the referenced integer.  So this:

		   local $/ = \32768; # or \"32768", or \$var_containing_32768
		   open my $fh, "<", $myfile or die $!;
		   local $_ = <$fh>;

	       will read a record of no more than 32768 bytes from FILE.  If you're not reading
	       from a record-oriented file (or your OS doesn't have record-oriented files), then
	       you'll likely get a full chunk of data with every read.	If a record is larger
	       than the record size you've set, you'll get the record back in pieces.  Trying to
	       set the record size to zero or less will cause reading in the (rest of the) whole

	       On VMS, record reads are done with the equivalent of "sysread", so it's best not
	       to mix record and non-record reads on the same file.  (This is unlikely to be a
	       problem, because any file you'd want to read in record mode is probably unusable
	       in line mode.)  Non-VMS systems do normal I/O, so it's safe to mix record and non-
	       record reads of a file.

	       See also "Newlines" in perlport.  Also see $..

       $|      If set to nonzero, forces a flush right away and after every write or print on the
	       currently selected output channel.  Default is 0 (regardless of whether the chan-
	       nel is really buffered by the system or not; $| tells you only whether you've
	       asked Perl explicitly to flush after each write).  STDOUT will typically be line
	       buffered if output is to the terminal and block buffered otherwise.  Setting this
	       variable is useful primarily when you are outputting to a pipe or socket, such as
	       when you are running a Perl program under rsh and want to see the output as it's
	       happening.  This has no effect on input buffering.  See "getc" in perlfunc for
	       that.  See "select" in perldoc on how to select the output channel.  See also
	       IO::Handle. (Mnemonic: when you want your pipes to be piping hot.)

       IO::Handle->output_field_separator EXPR
       $,      The output field separator for the print operator.  If defined, this value is
	       printed between each of print's arguments.  Default is "undef".	(Mnemonic: what
	       is printed when there is a "," in your print statement.)

       IO::Handle->output_record_separator EXPR
       $\      The output record separator for the print operator.  If defined, this value is
	       printed after the last of print's arguments.  Default is "undef".  (Mnemonic: you
	       set "$\" instead of adding "\n" at the end of the print.  Also, it's just like $/,
	       but it's what you get "back" from Perl.)

       $"      This is like $, except that it applies to array and slice values interpolated into
	       a double-quoted string (or similar interpreted string).	Default is a space.
	       (Mnemonic: obvious, I think.)

       $;      The subscript separator for multidimensional array emulation.  If you refer to a
	       hash element as


	       it really means

		   $foo{join($;, $a, $b, $c)}

	       But don't put

		   @foo{$a,$b,$c}      # a slice--note the @

	       which means


	       Default is "\034", the same as SUBSEP in awk.  If your keys contain binary data
	       there might not be any safe value for $;.  (Mnemonic: comma (the syntactic sub-
	       script separator) is a semi-semicolon.  Yeah, I know, it's pretty lame, but $, is
	       already taken for something more important.)

	       Consider using "real" multidimensional arrays as described in perllol.

       $#      The output format for printed numbers.  This variable is a half-hearted attempt to
	       emulate awk's OFMT variable.  There are times, however, when awk and Perl have
	       differing notions of what counts as numeric.  The initial value is "%.ng", where n
	       is the value of the macro DBL_DIG from your system's float.h.  This is different
	       from awk's default OFMT setting of "%.6g", so you need to set $# explicitly to get
	       awk's value.  (Mnemonic: # is the number sign.)

	       Use of $# is deprecated.

       $%      The current page number of the currently selected output channel.  Used with for-
	       mats.  (Mnemonic: % is page number in nroff.)

       $=      The current page length (printable lines) of the currently selected output chan-
	       nel.  Default is 60.  Used with formats.  (Mnemonic: = has horizontal lines.)

       $-      The number of lines left on the page of the currently selected output channel.
	       Used with formats.  (Mnemonic: lines_on_page - lines_printed.)

       @-      $-[0] is the offset of the start of the last successful match.  "$-["n"]" is the
	       offset of the start of the substring matched by n-th subpattern, or undef if the
	       subpattern did not match.

	       Thus after a match against $_, $& coincides with "substr $_, $-[0], $+[0] -
	       $-[0]".	Similarly, $n coincides with "substr $_, $-[n], $+[n] - $-[n]" if "$-[n]"
	       is defined, and $+ coincides with "substr $_, $-[$#-], $+[$#-] - $-[$#-]".  One
	       can use "$#-" to find the last matched subgroup in the last successful match.
	       Contrast with $#+, the number of subgroups in the regular expression.  Compare
	       with "@+".

	       This array holds the offsets of the beginnings of the last successful submatches
	       in the currently active dynamic scope.  "$-[0]" is the offset into the string of
	       the beginning of the entire match.  The nth element of this array holds the offset
	       of the nth submatch, so "$-[1]" is the offset where $1 begins, "$-[2]" the offset
	       where $2 begins, and so on.

	       After a match against some variable $var:

	       $` is the same as "substr($var, 0, $-[0])"
	       $& is the same as "substr($var, $-[0], $+[0] - $-[0])"
	       $' is the same as "substr($var, $+[0])"
	       $1 is the same as "substr($var, $-[1], $+[1] - $-[1])"
	       $2 is the same as "substr($var, $-[2], $+[2] - $-[2])"
	       $3 is the same as "substr($var, $-[3], $+[3] - $-[3])"
       $~      The name of the current report format for the currently selected output channel.
	       Default is the name of the filehandle.  (Mnemonic: brother to $^.)

       $^      The name of the current top-of-page format for the currently selected output chan-
	       nel.  Default is the name of the filehandle with _TOP appended.	(Mnemonic: points
	       to top of page.)

       IO::Handle->format_line_break_characters EXPR
       $:      The current set of characters after which a string may be broken to fill continua-
	       tion fields (starting with ^) in a format.  Default is " \n-", to break on white-
	       space or hyphens.  (Mnemonic: a "colon" in poetry is a part of a line.)

       IO::Handle->format_formfeed EXPR
       $^L     What formats output as a form feed.  Default is \f.

       $^A     The current value of the write() accumulator for format() lines.  A format con-
	       tains formline() calls that put their result into $^A.  After calling its format,
	       write() prints out the contents of $^A and empties.  So you never really see the
	       contents of $^A unless you call formline() yourself and then look at it.  See
	       perlform and "formline()" in perlfunc.

       $?      The status returned by the last pipe close, backtick (``) command, successful call
	       to wait() or waitpid(), or from the system() operator.  This is just the 16-bit
	       status word returned by the traditional Unix wait() system call (or else is made
	       up to look like it).  Thus, the exit value of the subprocess is really ("$? >>
	       8"), and "$? & 127" gives which signal, if any, the process died from, and "$? &
	       128" reports whether there was a core dump.  (Mnemonic: similar to sh and ksh.)

	       Additionally, if the "h_errno" variable is supported in C, its value is returned
	       via $? if any "gethost*()" function fails.

	       If you have installed a signal handler for "SIGCHLD", the value of $? will usually
	       be wrong outside that handler.

	       Inside an "END" subroutine $? contains the value that is going to be given to
	       "exit()".  You can modify $? in an "END" subroutine to change the exit status of
	       your program.  For example:

		   END {
		       $? = 1 if $? == 255;  # die would make it 255

	       Under VMS, the pragma "use vmsish 'status'" makes $? reflect the actual VMS exit
	       status, instead of the default emulation of POSIX status; see "$?" in perlvms for

	       Also see "Error Indicators".

	       The native status returned by the last pipe close, backtick (``) command, success-
	       ful call to wait() or waitpid(), or from the system() operator.	On POSIX-like
	       systems this value can be decoded with the WIFEXITED, WEXITSTATUS, WIFSIGNALED,
	       WTERMSIG, WIFSTOPPED, WSTOPSIG and WIFCONTINUED functions provided by the POSIX

	       Under VMS this reflects the actual VMS exit status; i.e. it is the same as $? when
	       the pragma "use vmsish 'status'" is in effect.

	       The object reference to the Encode object that is used to convert the source code
	       to Unicode.  Thanks to this variable your perl script does not have to be written
	       in UTF-8.  Default is undef.  The direct manipulation of this variable is highly

       $!      If used numerically, yields the current value of the C "errno" variable, or in
	       other words, if a system or library call fails, it sets this variable.  This means
	       that the value of $! is meaningful only immediately after a failure:

		   if (open my $fh, "<", $filename) {
		       # Here $! is meaningless.
		   } else {
		       # ONLY here is $! meaningful.
		       # Already here $! might be meaningless.
		   # Since here we might have either success or failure,
		   # here $! is meaningless.

	       In the above meaningless stands for anything: zero, non-zero, "undef".  A success-
	       ful system or library call does not set the variable to zero.

	       If used as a string, yields the corresponding system error string.  You can assign
	       a number to $! to set errno if, for instance, you want "$!" to return the string
	       for error n, or you want to set the exit value for the die() operator.  (Mnemonic:
	       What just went bang?)

	       Also see "Error Indicators".

       %!      Each element of "%!" has a true value only if $! is set to that value.  For exam-
	       ple, $!{ENOENT} is true if and only if the current value of $! is "ENOENT"; that
	       is, if the most recent error was "No such file or directory" (or its moral equiva-
	       lent: not all operating systems give that exact error, and certainly not all lan-
	       guages).  To check if a particular key is meaningful on your system, use "exists
	       $!{the_key}"; for a list of legal keys, use "keys %!".  See Errno for more infor-
	       mation, and also see above for the validity of $!.

       $^E     Error information specific to the current operating system.  At the moment, this
	       differs from $! under only VMS, OS/2, and Win32 (and for MacPerl).  On all other
	       platforms, $^E is always just the same as $!.

	       Under VMS, $^E provides the VMS status value from the last system error.  This is
	       more specific information about the last system error than that provided by $!.
	       This is particularly important when $! is set to EVMSERR.

	       Under OS/2, $^E is set to the error code of the last call to OS/2 API either via
	       CRT, or directly from perl.

	       Under Win32, $^E always returns the last error information reported by the Win32
	       call "GetLastError()" which describes the last error from within the Win32 API.
	       Most Win32-specific code will report errors via $^E.  ANSI C and Unix-like calls
	       set "errno" and so most portable Perl code will report errors via $!.

	       Caveats mentioned in the description of $! generally apply to $^E, also.
	       (Mnemonic: Extra error explanation.)

	       Also see "Error Indicators".

       $@      The Perl syntax error message from the last eval() operator.  If $@ is the null
	       string, the last eval() parsed and executed correctly (although the operations you
	       invoked may have failed in the normal fashion).	(Mnemonic: Where was the syntax
	       error "at"?)

	       Warning messages are not collected in this variable.  You can, however, set up a
	       routine to process warnings by setting $SIG{__WARN__} as described below.

	       Also see "Error Indicators".

       $$      The process number of the Perl running this script.  You should consider this
	       variable read-only, although it will be altered across fork() calls.  (Mnemonic:
	       same as shells.)

	       Note for Linux users: on Linux, the C functions "getpid()" and "getppid()" return
	       different values from different threads. In order to be portable, this behavior is
	       not reflected by $$, whose value remains consistent across threads. If you want to
	       call the underlying "getpid()", you may use the CPAN module "Linux::Pid".

       $<      The real uid of this process.  (Mnemonic: it's the uid you came from, if you're
	       running setuid.)  You can change both the real uid and the effective uid at the
	       same time by using POSIX::setuid().  Since changes to $< require a system call,
	       check $! after a change attempt to detect any possible errors.

       $>      The effective uid of this process.  Example:

		   $< = $>;	       # set real to effective uid
		   ($<,$>) = ($>,$<);  # swap real and effective uid

	       You can change both the effective uid and the real uid at the same time by using
	       POSIX::setuid().  Changes to $> require a check to $!  to detect any possible
	       errors after an attempted change.

	       (Mnemonic: it's the uid you went to, if you're running setuid.)	$< and $> can be
	       swapped only on machines supporting setreuid().

       $(      The real gid of this process.  If you are on a machine that supports membership in
	       multiple groups simultaneously, gives a space separated list of groups you are in.
	       The first number is the one returned by getgid(), and the subsequent ones by get-
	       groups(), one of which may be the same as the first number.

	       However, a value assigned to $( must be a single number used to set the real gid.
	       So the value given by $( should not be assigned back to $( without being forced
	       numeric, such as by adding zero. Note that this is different to the effective gid
	       ($)) which does take a list.

	       You can change both the real gid and the effective gid at the same time by using
	       POSIX::setgid().  Changes to $( require a check to $!  to detect any possible
	       errors after an attempted change.

	       (Mnemonic: parentheses are used to group things.  The real gid is the group you
	       left, if you're running setgid.)

       $)      The effective gid of this process.  If you are on a machine that supports member-
	       ship in multiple groups simultaneously, gives a space separated list of groups you
	       are in.	The first number is the one returned by getegid(), and the subsequent
	       ones by getgroups(), one of which may be the same as the first number.

	       Similarly, a value assigned to $) must also be a space-separated list of numbers.
	       The first number sets the effective gid, and the rest (if any) are passed to set-
	       groups().  To get the effect of an empty list for setgroups(), just repeat the new
	       effective gid; that is, to force an effective gid of 5 and an effectively empty
	       setgroups() list, say " $) = "5 5" ".

	       You can change both the effective gid and the real gid at the same time by using
	       POSIX::setgid() (use only a single numeric argument).  Changes to $) require a
	       check to $! to detect any possible errors after an attempted change.

	       (Mnemonic: parentheses are used to group things.  The effective gid is the group
	       that's right for you, if you're running setgid.)

	       $<, $>, $( and $) can be set only on machines that support the corresponding
	       set[re][ug]id() routine.  $( and $) can be swapped only on machines supporting

       $0      Contains the name of the program being executed.

	       On some (read: not all) operating systems assigning to $0 modifies the argument
	       area that the "ps" program sees.  On some platforms you may have to use special
	       "ps" options or a different "ps" to see the changes.  Modifying the $0 is more
	       useful as a way of indicating the current program state than it is for hiding the
	       program you're running.	(Mnemonic: same as sh and ksh.)

	       Note that there are platform specific limitations on the maximum length of $0.  In
	       the most extreme case it may be limited to the space occupied by the original $0.

	       In some platforms there may be arbitrary amount of padding, for example space
	       characters, after the modified name as shown by "ps".  In some platforms this pad-
	       ding may extend all the way to the original length of the argument area, no matter
	       what you do (this is the case for example with Linux 2.2).

	       Note for BSD users: setting $0 does not completely remove "perl" from the ps(1)
	       output.	For example, setting $0 to "foobar" may result in "perl: foobar (perl)"
	       (whether both the "perl: " prefix and the " (perl)" suffix are shown depends on
	       your exact BSD variant and version).  This is an operating system feature, Perl
	       cannot help it.

	       In multithreaded scripts Perl coordinates the threads so that any thread may mod-
	       ify its copy of the $0 and the change becomes visible to ps(1) (assuming the oper-
	       ating system plays along).  Note that the view of $0 the other threads have will
	       not change since they have their own copies of it.

       $[      The index of the first element in an array, and of the first character in a sub-
	       string.	Default is 0, but you could theoretically set it to 1 to make Perl behave
	       more like awk (or Fortran) when subscripting and when evaluating the index() and
	       substr() functions.  (Mnemonic: [ begins subscripts.)

	       As of release 5 of Perl, assignment to $[ is treated as a compiler directive, and
	       cannot influence the behavior of any other file.  (That's why you can only assign
	       compile-time constants to it.)  Its use is highly discouraged.

	       Note that, unlike other compile-time directives (such as strict), assignment to $[
	       can be seen from outer lexical scopes in the same file.	However, you can use
	       local() on it to strictly bind its value to a lexical block.

       $]      The version + patchlevel / 1000 of the Perl interpreter.  This variable can be
	       used to determine whether the Perl interpreter executing a script is in the right
	       range of versions.  (Mnemonic: Is this version of perl in the right bracket?)

		   warn "No checksumming!\n" if $] < 3.019;

	       See also the documentation of "use VERSION" and "require VERSION" for a convenient
	       way to fail if the running Perl interpreter is too old.

	       When testing the variable, to steer clear of floating point inaccuracies you might
	       want to prefer the inequality tests "<" and ">" to the tests containing equiva-
	       lence: "<=", "==", and ">=".

	       The floating point representation can sometimes lead to inaccurate numeric compar-
	       isons.  See $^V for a more modern representation of the Perl version that allows
	       accurate string comparisons.

       $^C     The current value of the flag associated with the -c switch.  Mainly of use with
	       -MO=... to allow code to alter its behavior when being compiled, such as for exam-
	       ple to AUTOLOAD at compile time rather than normal, deferred loading.  Setting
	       "$^C = 1" is similar to calling "B::minus_c".

       $^D     The current value of the debugging flags.  (Mnemonic: value of -D switch.) May be
	       read or set. Like its command-line equivalent, you can use numeric or symbolic
	       values, eg "$^D = 10" or "$^D = "st"".

       $^F     The maximum system file descriptor, ordinarily 2.  System file descriptors are
	       passed to exec()ed processes, while higher file descriptors are not.  Also, during
	       an open(), system file descriptors are preserved even if the open() fails.  (Ordi-
	       nary file descriptors are closed before the open() is attempted.)  The close-on-
	       exec status of a file descriptor will be decided according to the value of $^F
	       when the corresponding file, pipe, or socket was opened, not the time of the

       $^H     WARNING: This variable is strictly for internal use only.  Its availability,
	       behavior, and contents are subject to change without notice.

	       This variable contains compile-time hints for the Perl interpreter.  At the end of
	       compilation of a BLOCK the value of this variable is restored to the value when
	       the interpreter started to compile the BLOCK.

	       When perl begins to parse any block construct that provides a lexical scope (e.g.,
	       eval body, required file, subroutine body, loop body, or conditional block), the
	       existing value of $^H is saved, but its value is left unchanged.  When the compi-
	       lation of the block is completed, it regains the saved value.  Between the points
	       where its value is saved and restored, code that executes within BEGIN blocks is
	       free to change the value of $^H.

	       This behavior provides the semantic of lexical scoping, and is used in, for
	       instance, the "use strict" pragma.

	       The contents should be an integer; different bits of it are used for different
	       pragmatic flags.  Here's an example:

		   sub add_100 { $^H |= 0x100 }

		   sub foo {
		       BEGIN { add_100() }

	       Consider what happens during execution of the BEGIN block.  At this point the
	       BEGIN block has already been compiled, but the body of foo() is still being com-
	       piled.  The new value of $^H will therefore be visible only while the body of
	       foo() is being compiled.

	       Substitution of the above BEGIN block with:

		   BEGIN { require strict; strict->import('vars') }

	       demonstrates how "use strict 'vars'" is implemented.  Here's a conditional version
	       of the same lexical pragma:

		   BEGIN { require strict; strict->import('vars') if $condition }

       %^H     WARNING: This variable is strictly for internal use only.  Its availability,
	       behavior, and contents are subject to change without notice.

	       The %^H hash provides the same scoping semantic as $^H.	This makes it useful for
	       implementation of lexically scoped pragmas.

       $^I     The current value of the inplace-edit extension.  Use "undef" to disable inplace
	       editing.  (Mnemonic: value of -i switch.)

       $^M     By default, running out of memory is an untrappable, fatal error.  However, if
	       suitably built, Perl can use the contents of $^M as an emergency memory pool after
	       die()ing.  Suppose that your Perl were compiled with "-DPERL_EMERGENCY_SBRK" and
	       used Perl's malloc.  Then

		   $^M = 'a' x (1 << 16);

	       would allocate a 64K buffer for use in an emergency.  See the INSTALL file in the
	       Perl distribution for information on how to add custom C compilation flags when
	       compiling perl.	To discourage casual use of this advanced feature, there is no
	       English long name for this variable.

       $^O     The name of the operating system under which this copy of Perl was built, as
	       determined during the configuration process.  The value is identical to $Con-
	       fig{'osname'}.  See also Config and the -V command-line switch documented in perl-

	       In Windows platforms, $^O is not very helpful: since it is always "MSWin32", it
	       doesn't tell the difference between 95/98/ME/NT/2000/XP/CE/.NET.  Use
	       Win32::GetOSName() or Win32::GetOSVersion() (see Win32 and perlport) to distin-
	       guish between the variants.

	       An internal variable used by PerlIO.  A string in two parts, separated by a "\0"
	       byte, the first part describes the input layers, the second part describes the
	       output layers.

       $^P     The internal variable for debugging support.  The meanings of the various bits are
	       subject to change, but currently indicate:

	       0x01  Debug subroutine enter/exit.

	       0x02  Line-by-line debugging. Causes DB::DB() subroutine to be called for each
		     statement executed. Also causes saving source code lines (like 0x400).

	       0x04  Switch off optimizations.

	       0x08  Preserve more data for future interactive inspections.

	       0x10  Keep info about source lines on which a subroutine is defined.

	       0x20  Start with single-step on.

	       0x40  Use subroutine address instead of name when reporting.

	       0x80  Report "goto &subroutine" as well.

	       0x100 Provide informative "file" names for evals based on the place they were com-

	       0x200 Provide informative names to anonymous subroutines based on the place they
		     were compiled.

	       0x400 Save source code lines into "@{"_<$filename"}".

	       Some bits may be relevant at compile-time only, some at run-time only.  This is a
	       new mechanism and the details may change.  See also perldebguts.

       $^R     The result of evaluation of the last successful "(?{ code })" regular expression
	       assertion (see perlre).	May be written to.

       $^S     Current state of the interpreter.

		   $^S	       State
		   ---------   -------------------
		   undef       Parsing module/eval
		   true (1)    Executing an eval
		   false (0)   Otherwise

	       The first state may happen in $SIG{__DIE__} and $SIG{__WARN__} handlers.

       $^T     The time at which the program began running, in seconds since the epoch (beginning
	       of 1970).  The values returned by the -M, -A, and -C filetests are based on this

	       Reflects if taint mode is on or off.  1 for on (the program was run with -T), 0
	       for off, -1 when only taint warnings are enabled (i.e. with -t or -TU).

	       Reflects certain Unicode settings of Perl.  See perlrun documentation for the "-C"
	       switch for more information about the possible values. This variable is set during
	       Perl startup and is thereafter read-only.

	       This variable controls the state of the internal UTF-8 offset caching code.  1 for
	       on (the default), 0 for off, -1 to debug the caching code by checking all its
	       results against linear scans, and panicking on any discrepancy.

	       This variable indicates whether an UTF-8 locale was detected by perl at startup.
	       This information is used by perl when it's in adjust-utf8ness-to-locale mode (as
	       when run with the "-CL" command-line switch); see perlrun for more info on this.

       $^V     The revision, version, and subversion of the Perl interpreter, represented as a
	       string composed of characters with those ordinals.  Thus in Perl v5.6.0 it equals
	       "chr(5) . chr(6) . chr(0)" and will return true for "$^V eq v5.6.0".  Note that
	       the characters in this string value can potentially be greater than 255.

	       This variable first appeared in perl 5.6.0; earlier versions of perl will see an
	       undefined value.

	       This can be used to determine whether the Perl interpreter executing a script is
	       in the right range of versions.	(Mnemonic: use ^V for Version Control.)  Example:

		   warn "Hashes not randomized!\n" if !$^V or $^V lt v5.8.1

	       To convert $^V into its string representation use sprintf()'s "%vd" conversion:

		   printf "version is v%vd\n", $^V;  # Perl's version

	       See the documentation of "use VERSION" and "require VERSION" for a convenient way
	       to fail if the running Perl interpreter is too old.

	       See also $] for an older representation of the Perl version.

       $^W     The current value of the warning switch, initially true if -w was used, false oth-
	       erwise, but directly modifiable.  (Mnemonic: related to the -w switch.)	See also

	       The current set of warning checks enabled by the "use warnings" pragma.	See the
	       documentation of "warnings" for more details.

	       If this variable is set to a true value, then stat() on Windows will not try to
	       open the file. This means that the link count cannot be determined and file
	       attributes may be out of date if additional hardlinks to the file exist. On the
	       other hand, not opening the file is considerably faster, especially for files on
	       network drives.

	       This variable could be set in the sitecustomize.pl file to configure the local
	       Perl installation to use "sloppy" stat() by default.  See perlrun for more infor-
	       mation about site customization.

       $^X     The name used to execute the current copy of Perl, from C's "argv[0]" or (where
	       supported) /proc/self/exe.

	       Depending on the host operating system, the value of $^X may be a relative or
	       absolute pathname of the perl program file, or may be the string used to invoke
	       perl but not the pathname of the perl program file.  Also, most operating systems
	       permit invoking programs that are not in the PATH environment variable, so there
	       is no guarantee that the value of $^X is in PATH.  For VMS, the value may or may
	       not include a version number.

	       You usually can use the value of $^X to re-invoke an independent copy of the same
	       perl that is currently running, e.g.,

		 @first_run = `$^X -le "print int rand 100 for 1..100"`;

	       But recall that not all operating systems support forking or capturing of the out-
	       put of commands, so this complex statement may not be portable.

	       It is not safe to use the value of $^X as a path name of a file, as some operating
	       systems that have a mandatory suffix on executable files do not require use of the
	       suffix when invoking a command.	To convert the value of $^X to a path name, use
	       the following statements:

		 # Build up a set of file names (not command names).
		 use Config;
		 $this_perl = $^X;
		 if ($^O ne 'VMS')
		    {$this_perl .= $Config{_exe}
			 unless $this_perl =~ m/$Config{_exe}$/i;}

	       Because many operating systems permit anyone with read access to the Perl program
	       file to make a copy of it, patch the copy, and then execute the copy, the secu-
	       rity-conscious Perl programmer should take care to invoke the installed copy of
	       perl, not the copy referenced by $^X.  The following statements accomplish this
	       goal, and produce a pathname that can be invoked as a command or referenced as a

		 use Config;
		 $secure_perl_path = $Config{perlpath};
		 if ($^O ne 'VMS')
		    {$secure_perl_path .= $Config{_exe}
			 unless $secure_perl_path =~ m/$Config{_exe}$/i;}

       ARGV    The special filehandle that iterates over command-line filenames in @ARGV. Usually
	       written as the null filehandle in the angle operator "<>". Note that currently
	       "ARGV" only has its magical effect within the "<>" operator; elsewhere it is just
	       a plain filehandle corresponding to the last file opened by "<>". In particular,
	       passing "\*ARGV" as a parameter to a function that expects a filehandle may not
	       cause your function to automatically read the contents of all the files in @ARGV.

       $ARGV   contains the name of the current file when reading from <>.

       @ARGV   The array @ARGV contains the command-line arguments intended for the script.
	       $#ARGV is generally the number of arguments minus one, because $ARGV[0] is the
	       first argument, not the program's command name itself.  See $0 for the command

       ARGVOUT The special filehandle that points to the currently open output file when doing
	       edit-in-place processing with -i.  Useful when you have to do a lot of inserting
	       and don't want to keep modifying $_.  See perlrun for the -i switch.

       @F      The array @F contains the fields of each line read in when autosplit mode is
	       turned on.  See perlrun for the -a switch.  This array is package-specific, and
	       must be declared or given a full package name if not in package main when running
	       under "strict 'vars'".

       @INC    The array @INC contains the list of places that the "do EXPR", "require", or "use"
	       constructs look for their library files.  It initially consists of the arguments
	       to any -I command-line switches, followed by the default Perl library, probably
	       /usr/local/lib/perl, followed by ".", to represent the current directory.  ("."
	       will not be appended if taint checks are enabled, either by "-T" or by "-t".)  If
	       you need to modify this at runtime, you should use the "use lib" pragma to get the
	       machine-dependent library properly loaded also:

		   use lib '/mypath/libdir/';
		   use SomeMod;

	       You can also insert hooks into the file inclusion system by putting Perl code
	       directly into @INC.  Those hooks may be subroutine references, array references or
	       blessed objects.  See "require" in perlfunc for details.

       @_      Within a subroutine the array @_ contains the parameters passed to that subrou-
	       tine.  See perlsub.

       %INC    The hash %INC contains entries for each filename included via the "do", "require",
	       or "use" operators.  The key is the filename you specified (with module names con-
	       verted to pathnames), and the value is the location of the file found.  The
	       "require" operator uses this hash to determine whether a particular file has
	       already been included.

	       If the file was loaded via a hook (e.g. a subroutine reference, see "require" in
	       perlfunc for a description of these hooks), this hook is by default inserted into
	       %INC in place of a filename.  Note, however, that the hook may have set the %INC
	       entry by itself to provide some more specific info.

	       The hash %ENV contains your current environment.  Setting a value in "ENV" changes
	       the environment for any child processes you subsequently fork() off.

	       The hash %SIG contains signal handlers for signals.  For example:

		   sub handler {       # 1st argument is signal name
		       my($sig) = @_;
		       print "Caught a SIG$sig--shutting down\n";

		   $SIG{'INT'}	= \&handler;
		   $SIG{'QUIT'} = \&handler;
		   $SIG{'INT'}	= 'DEFAULT';   # restore default action
		   $SIG{'QUIT'} = 'IGNORE';    # ignore SIGQUIT

	       Using a value of 'IGNORE' usually has the effect of ignoring the signal, except
	       for the "CHLD" signal.  See perlipc for more about this special case.

	       Here are some other examples:

		   $SIG{"PIPE"} = "Plumber";   # assumes main::Plumber (not recommended)
		   $SIG{"PIPE"} = \&Plumber;   # just fine; assume current Plumber
		   $SIG{"PIPE"} = *Plumber;    # somewhat esoteric
		   $SIG{"PIPE"} = Plumber();   # oops, what did Plumber() return??

	       Be sure not to use a bareword as the name of a signal handler, lest you inadver-
	       tently call it.

	       If your system has the sigaction() function then signal handlers are installed
	       using it.  This means you get reliable signal handling.

	       The default delivery policy of signals changed in Perl 5.8.0 from immediate (also
	       known as "unsafe") to deferred, also known as "safe signals".  See perlipc for
	       more information.

	       Certain internal hooks can be also set using the %SIG hash.  The routine indicated
	       by $SIG{__WARN__} is called when a warning message is about to be printed.  The
	       warning message is passed as the first argument.  The presence of a "__WARN__"
	       hook causes the ordinary printing of warnings to "STDERR" to be suppressed.  You
	       can use this to save warnings in a variable, or turn warnings into fatal errors,
	       like this:

		   local $SIG{__WARN__} = sub { die $_[0] };
		   eval $proggie;

	       As the 'IGNORE' hook is not supported by "__WARN__", you can disable warnings
	       using the empty subroutine:

		   local $SIG{__WARN__} = sub {};

	       The routine indicated by $SIG{__DIE__} is called when a fatal exception is about
	       to be thrown.  The error message is passed as the first argument.  When a
	       "__DIE__" hook routine returns, the exception processing continues as it would
	       have in the absence of the hook, unless the hook routine itself exits via a
	       "goto", a loop exit, or a "die()".  The "__DIE__" handler is explicitly disabled
	       during the call, so that you can die from a "__DIE__" handler.  Similarly for

	       Due to an implementation glitch, the $SIG{__DIE__} hook is called even inside an
	       eval().	Do not use this to rewrite a pending exception in $@, or as a bizarre
	       substitute for overriding "CORE::GLOBAL::die()".  This strange action at a dis-
	       tance may be fixed in a future release so that $SIG{__DIE__} is only called if
	       your program is about to exit, as was the original intent.  Any other use is dep-

	       "__DIE__"/"__WARN__" handlers are very special in one respect: they may be called
	       to report (probable) errors found by the parser.  In such a case the parser may be
	       in inconsistent state, so any attempt to evaluate Perl code from such a handler
	       will probably result in a segfault.  This means that warnings or errors that
	       result from parsing Perl should be used with extreme caution, like this:

		   require Carp if defined $^S;
		   Carp::confess("Something wrong") if defined &Carp::confess;
		   die "Something wrong, but could not load Carp to give backtrace...
			To see backtrace try starting Perl with -MCarp switch";

	       Here the first line will load Carp unless it is the parser who called the handler.
	       The second line will print backtrace and die if Carp was available.  The third
	       line will be executed only if Carp was not available.

	       See "die" in perlfunc, "warn" in perlfunc, "eval" in perlfunc, and warnings for
	       additional information.

       Error Indicators

       The variables $@, $!, $^E, and $? contain information about different types of error con-
       ditions that may appear during execution of a Perl program.  The variables are shown
       ordered by the "distance" between the subsystem which reported the error and the Perl
       process.  They correspond to errors detected by the Perl interpreter, C library, operating
       system, or an external program, respectively.

       To illustrate the differences between these variables, consider the following Perl expres-
       sion, which uses a single-quoted string:

	   eval q{
	       open my $pipe, "/cdrom/install |" or die $!;
	       my @res = <$pipe>;
	       close $pipe or die "bad pipe: $?, $!";

       After execution of this statement all 4 variables may have been set.

       $@ is set if the string to be "eval"-ed did not compile (this may happen if "open" or
       "close" were imported with bad prototypes), or if Perl code executed during evaluation
       die()d .  In these cases the value of $@ is the compile error, or the argument to "die"
       (which will interpolate $! and $?).  (See also Fatal, though.)

       When the eval() expression above is executed, open(), "<PIPE>", and "close" are translated
       to calls in the C run-time library and thence to the operating system kernel.  $! is set
       to the C library's "errno" if one of these calls fails.

       Under a few operating systems, $^E may contain a more verbose error indicator, such as in
       this case, "CDROM tray not closed."  Systems that do not support extended error messages
       leave $^E the same as $!.

       Finally, $? may be set to non-0 value if the external program /cdrom/install fails.  The
       upper eight bits reflect specific error conditions encountered by the program (the pro-
       gram's exit() value).   The lower eight bits reflect mode of failure, like signal death
       and core dump information  See wait(2) for details.  In contrast to $! and $^E, which are
       set only if error condition is detected, the variable $? is set on each "wait" or pipe
       "close", overwriting the old value.  This is more like $@, which on every eval() is always
       set on failure and cleared on success.

       For more details, see the individual descriptions at $@, $!, $^E, and $?.

       Technical Note on the Syntax of Variable Names

       Variable names in Perl can have several formats.  Usually, they must begin with a letter
       or underscore, in which case they can be arbitrarily long (up to an internal limit of 251
       characters) and may contain letters, digits, underscores, or the special sequence "::" or
       "'".  In this case, the part before the last "::" or "'" is taken to be a package quali-
       fier; see perlmod.

       Perl variable names may also be a sequence of digits or a single punctuation or control
       character.  These names are all reserved for special uses by Perl; for example, the all-
       digits names are used to hold data captured by backreferences after a regular expression
       match.  Perl has a special syntax for the single-control-character names: It understands
       "^X" (caret "X") to mean the control-"X" character.  For example, the notation $^W (dol-
       lar-sign caret "W") is the scalar variable whose name is the single character control-"W".
       This is better than typing a literal control-"W" into your program.

       Finally, new in Perl 5.6, Perl variable names may be alphanumeric strings that begin with
       control characters (or better yet, a caret).  These variables must be written in the form
       "${^Foo}"; the braces are not optional.	"${^Foo}" denotes the scalar variable whose name
       is a control-"F" followed by two "o"'s.	These variables are reserved for future special
       uses by Perl, except for the ones that begin with "^_" (control-underscore or caret-under-
       score).	No control-character name that begins with "^_" will acquire a special meaning in
       any future version of Perl; such names may therefore be used safely in programs.  $^_
       itself, however, is reserved.

       Perl identifiers that begin with digits, control characters, or punctuation characters are
       exempt from the effects of the "package" declaration and are always forced to be in pack-
       age "main"; they are also exempt from "strict 'vars'" errors.  A few other names are also
       exempt in these ways:

	       ENV	       STDIN
	       INC	       STDOUT
	       ARGV	       STDERR
	       ARGVOUT	       _

       In particular, the new special "${^_XYZ}" variables are always taken to be in package
       "main", regardless of any "package" declarations presently in scope.

       Due to an unfortunate accident of Perl's implementation, "use English" imposes a consider-
       able performance penalty on all regular expression matches in a program, regardless of
       whether they occur in the scope of "use English".  For that reason, saying "use English"
       in libraries is strongly discouraged.  See the Devel::SawAmpersand module documentation
       from CPAN ( http://www.cpan.org/modules/by-module/Devel/ ) for more information. Writing
       "use English '-no_match_vars';" avoids the performance penalty.

       Having to even think about the $^S variable in your exception handlers is simply wrong.
       $SIG{__DIE__} as currently implemented invites grievous and difficult to track down
       errors.	Avoid it and use an "END{}" or CORE::GLOBAL::die override instead.

perl v5.8.9				    2007-11-17				       PERLVAR(1)

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