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

NAME
       perlvar - Perl predefined variables

DESCRIPTION
   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
       qualifier; 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
       (dollar-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.

       Since 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-
       underscore).  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
       package "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
	   SIG

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

SPECIAL VARIABLES
       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
       current package.  Some even have medium names, generally borrowed from awk.  To avoid a
       performance hit, if you don't need the $PREMATCH, $MATCH, or $POSTMATCH it's best to use
       the "English" module without them:

	   use English '-no_match_vars';

       Before you continue, note the sort order for variables.	In general, we first list the
       variables in case-insensitive, almost-lexigraphical order (ignoring the "{" or "^"
       preceding words, as in "${^UNICODE}" or $^T), although $_ and @_ move up to the top of the
       pile.  For variables with the same identifier, we list it in order of scalar, array, hash,
       and bareword.

   General Variables
       $ARG
       $_      The default input and pattern-searching space.  The following pairs are
	       equivalent:

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

		   /^Subject:/
		   $_ =~ /^Subject:/

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

		   chomp
		   chomp($_)

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

	       o  The following functions use $_ as a default argument:

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

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

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

	       o  The default iterator variable in a "foreach" loop if no other variable is
		  supplied.

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

	       o  The implicit variable of "given()".

	       o  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.

	       As $_ is a global variable, this may lead in some cases to unwanted side-effects.
	       As of perl 5.10, you can now use a lexical version of $_ by declaring it in a file
	       or in a block with "my".  Moreover, declaring "our $_" restores the global $_ in
	       the current scope.

	       Mnemonic: underline is understood in certain operations.

       @ARG
       @_      Within a subroutine the array @_ contains the parameters passed to that
	       subroutine.  Inside a subroutine, @_ is the default array for the array operators
	       "push", "pop", "shift", and "unshift".

	       See perlsub.

       $LIST_SEPARATOR
       $"      When an array or an array slice is interpolated into a double-quoted string or a
	       similar context such as "/.../", its elements are separated by this value.
	       Default is a space.  For example, this:

		   print "The array is: @array\n";

	       is equivalent to this:

		   print "The array is: " . join($", @array) . "\n";

	       Mnemonic: works in double-quoted context.

       $PROCESS_ID
       $PID
       $$      The process number of the Perl running this script.  Though you can set this
	       variable, doing so is generally discouraged, although it can be invaluable for
	       some testing purposes.  It will be reset automatically across "fork()" calls.

	       Note for Linux and Debian GNU/kFreeBSD users: Before Perl v5.16.0 perl would
	       emulate POSIX semantics on Linux systems using LinuxThreads, a partial
	       implementation of POSIX Threads that has since been superseded by the Native POSIX
	       Thread Library (NPTL).

	       LinuxThreads is now obsolete on Linux, and and caching "getpid()" like this made
	       embedding perl unnecessarily complex (since you'd have to manually update the
	       value of $$), so now $$ and "getppid()" will always return the same values as the
	       underlying C library.

	       Debian GNU/kFreeBSD systems also used LinuxThreads up until and including the 6.0
	       release, but after that moved to FreeBSD thread semantics, which are POSIX-like.

	       To see if your system is affected by this discrepancy check if "getconf
	       GNU_LIBPTHREAD_VERSION | grep -q NPTL" returns a false value. NTPL threads
	       preserve the POSIX semantics.

	       Mnemonic: same as shells.

       $PROGRAM_NAME
       $0      Contains the name of the program being executed.

	       On some (but 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.

	       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
	       padding 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
	       modify its copy of the $0 and the change becomes visible to ps(1) (assuming the
	       operating system plays along).  Note that the view of $0 the other threads have
	       will not change since they have their own copies of it.

	       If the program has been given to perl via the switches "-e" or "-E", $0 will
	       contain the string "-e".

	       On Linux as of perl 5.14 the legacy process name will be set with prctl(2), in
	       addition to altering the POSIX name via "argv[0]" as perl has done since version
	       4.000.  Now system utilities that read the legacy process name such as ps, top and
	       killall will recognize the name you set when assigning to $0.  The string you
	       supply will be cut off at 16 bytes, this is a limitation imposed by Linux.

	       Mnemonic: same as sh and ksh.

       $REAL_GROUP_ID
       $GID
       $(      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
	       "getgroups()", 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.

       $EFFECTIVE_GROUP_ID
       $EGID
       $)      The effective 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 "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
	       "setgroups()".  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.

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

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

       $REAL_USER_ID
       $UID
       $<      The real uid of this process.  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.

	       Mnemonic: it's the uid you came from, if you're running setuid.

       $EFFECTIVE_USER_ID
       $EUID
       $>      The effective uid of this process.  For example:

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

	       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.

	       $< and $> can be swapped only on machines supporting "setreuid()".

	       Mnemonic: it's the uid you went to, if you're running setuid.

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

		   $foo{$a,$b,$c}

	       it really means

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

	       But don't put

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

	       which means

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

	       Default is "\034", the same as SUBSEP in awk.  If your keys contain binary data
	       there might not be any safe value for $;.

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

	       Mnemonic: comma (the syntactic subscript separator) is a semi-semicolon.

       $a
       $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 function.

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

       $SYSTEM_FD_MAX
       $^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 (ordinary 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 "exec()".

       @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.

       %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
	       converted 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.

       $INPLACE_EDIT
       $^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.

	       This variable was added in Perl 5.004.

       $OSNAME
       $^O     The name of the operating system under which this copy of Perl was built, as
	       determined during the configuration process.  For examples see "PLATFORMS" in
	       perlport.

	       The value is identical to $Config{'osname'}.  See also Config and the -V command-
	       line switch documented in perlrun.

	       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
	       distinguish between the variants.

	       This variable was added in Perl 5.003.

       %SIG    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";
		       close(LOG);
		       exit(0);
		       }

		   $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
	       inadvertently 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
	       &sub", 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
	       "__WARN__".

	       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
	       distance 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
	       deprecated.

	       "__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...\n\t"
		     . "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.

	       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.

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

       $BASETIME
       $^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
	       value.

       $PERL_VERSION
       $^V     The revision, version, and subversion of the Perl interpreter, represented as a
	       "version" object.

	       This variable first appeared in perl 5.6.0; earlier versions of perl will see an
	       undefined value.  Before perl 5.10.0 $^V was represented as a v-string.

	       $^V can be used to determine whether the Perl interpreter executing a script is in
	       the right range of versions.  For 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.

	       This variable was added in Perl 5.6.

	       Mnemonic: use ^V for Version Control.

       ${^WIN32_SLOPPY_STAT}
	       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 the documentation for
	       -f in perlrun for more information about site customization.

	       This variable was added in Perl 5.10.

       $EXECUTABLE_NAME
       $^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
	       output 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;
		   my $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
	       security-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
	       file.

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

   Variables related to regular expressions
       Most of the special variables related to regular expressions are side effects.  Perl sets
       these variables when it has a successful match, so you should check the match result
       before using them.  For instance:

	   if( /P(A)TT(ER)N/ ) {
	       print "I found $1 and $2\n";
	       }

       These variables are read-only and dynamically-scoped, unless we note otherwise.

       The dynamic nature of the regular expression variables means that their value is limited
       to the block that they are in, as demonstrated by this bit of code:

	   my $outer = 'Wallace and Grommit';
	   my $inner = 'Mutt and Jeff';

	   my $pattern = qr/(\S+) and (\S+)/;

	   sub show_n { print "\$1 is $1; \$2 is $2\n" }

	   {
	   OUTER:
	       show_n() if $outer =~ m/$pattern/;

	       INNER: {
		   show_n() if $inner =~ m/$pattern/;
		   }

	       show_n();
	   }

       The output shows that while in the "OUTER" block, the values of $1 and $2 are from the
       match against $outer.  Inside the "INNER" block, the values of $1 and $2 are from the
       match against $inner, but only until the end of the block (i.e. the dynamic scope).  After
       the "INNER" block completes, the values of $1 and $2 return to the values for the match
       against $outer even though we have not made another match:

	   $1 is Wallace; $2 is Grommit
	   $1 is Mutt; $2 is Jeff
	   $1 is Wallace; $2 is Grommit

       Due to an unfortunate accident of Perl's implementation, "use English" imposes a
       considerable performance penalty on all regular expression matches in a program because it
       uses the "$`", $&, and "$'", regardless of whether they occur in the scope of "use
       English".  For that reason, saying "use English" in libraries is strongly discouraged
       unless you import it without the match variables:

	   use English '-no_match_vars'

       The "Devel::NYTProf" and "Devel::FindAmpersand" modules can help you find uses of these
       problematic match variables in your code.

       Since Perl 5.10, you can use the "/p" match operator flag and the "${^PREMATCH}",
       "${^MATCH}", and "${^POSTMATCH}" variables instead so you only suffer the performance
       penalties.

       $<digits> ($1, $2, ...)
	       Contains the subpattern from the corresponding set of capturing parentheses from
	       the last successful pattern match, not counting patterns matched in nested blocks
	       that have been exited already.

	       These variables are read-only and dynamically-scoped.

	       Mnemonic: like \digits.

       $MATCH
       $&      The string matched by the last successful pattern match (not counting any matches
	       hidden within a BLOCK or "eval()" enclosed by the current BLOCK).

	       The use of this variable anywhere in a program imposes a considerable performance
	       penalty on all regular expression matches.  To avoid this penalty, you can extract
	       the same substring by using "@-".  Starting with Perl 5.10, you can use the "/p"
	       match flag and the "${^MATCH}" variable to do the same thing for particular match
	       operations.

	       This variable is read-only and dynamically-scoped.

	       Mnemonic: like "&" in some editors.

       ${^MATCH}
	       This is similar to $& ($MATCH) except that it does not incur the performance
	       penalty associated with that variable, and is only guaranteed to return a defined
	       value when the pattern was compiled or executed with the "/p" modifier.

	       This variable was added in Perl 5.10.

	       This variable is read-only and dynamically-scoped.

       $PREMATCH
       $`      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.

	       The use of this variable anywhere in a program imposes a considerable performance
	       penalty on all regular expression matches.  To avoid this penalty, you can extract
	       the same substring by using "@-".  Starting with Perl 5.10, you can use the "/p"
	       match flag and the "${^PREMATCH}" variable to do the same thing for particular
	       match operations.

	       This variable is read-only and dynamically-scoped.

	       Mnemonic: "`" often precedes a quoted string.

       ${^PREMATCH}
	       This is similar to "$`" ($PREMATCH) except that it does not incur the performance
	       penalty associated with that variable, and is only guaranteed to return a defined
	       value when the pattern was compiled or executed with the "/p" modifier.

	       This variable was added in Perl 5.10

	       This variable is read-only and dynamically-scoped.

       $POSTMATCH
       $'      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).	Example:

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

	       The use of this variable anywhere in a program imposes a considerable performance
	       penalty on all regular expression matches.  To avoid this penalty, you can extract
	       the same substring by using "@-".  Starting with Perl 5.10, you can use the "/p"
	       match flag and the "${^POSTMATCH}" variable to do the same thing for particular
	       match operations.

	       This variable is read-only and dynamically-scoped.

	       Mnemonic: "'" often follows a quoted string.

       ${^POSTMATCH}
	       This is similar to "$'" ($POSTMATCH) except that it does not incur the performance
	       penalty associated with that variable, and is only guaranteed to return a defined
	       value when the pattern was compiled or executed with the "/p" modifier.

	       This variable was added in Perl 5.10.

	       This variable is read-only and dynamically-scoped.

       $LAST_PAREN_MATCH
       $+      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 = $+);

	       This variable is read-only and dynamically-scoped.

	       Mnemonic: be positive and forward looking.

       $LAST_SUBMATCH_RESULT
       $^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.

	       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 was added in Perl 5.8.

	       Mnemonic: the (possibly) Nested parenthesis that most recently closed.

       @LAST_MATCH_END
       @+      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.

	       This variable was added in Perl 5.6.

       %LAST_PAREN_MATCH
       %+      Similar to "@+", the "%+" hash allows access to the named capture buffers, should
	       they exist, in the last successful match in the currently active dynamic scope.

	       For example, $+{foo} is equivalent to $1 after the following match:

		   'foo' =~ /(?<foo>foo)/;

	       The keys of the "%+" hash list only the names of buffers that have captured (and
	       that are thus associated to defined values).

	       The underlying behaviour of "%+" is provided by the Tie::Hash::NamedCapture
	       module.

	       Note: "%-" and "%+" are tied views into a common internal hash associated with the
	       last successful regular expression.  Therefore mixing iterative access to them via
	       "each" may have unpredictable results.  Likewise, if the last successful match
	       changes, then the results may be surprising.

	       This variable was added in Perl 5.10.

	       This variable is read-only and dynamically-scoped.

       @LAST_MATCH_START
       @-      "$-[0]" is the offset of the start of the last successful match.  "$-["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])"

	       This variable was added in Perl 5.6.

       %LAST_MATCH_START
       %-      Similar to "%+", this variable allows access to the named capture groups in the
	       last successful match in the currently active dynamic scope.  To each capture
	       group name found in the regular expression, it associates a reference to an array
	       containing the list of values captured by all buffers with that name (should there
	       be several of them), in the order where they appear.

	       Here's an example:

		   if ('1234' =~ /(?<A>1)(?<B>2)(?<A>3)(?<B>4)/) {
		       foreach my $bufname (sort keys %-) {
			   my $ary = $-{$bufname};
			   foreach my $idx (0..$#$ary) {
			       print "\$-{$bufname}[$idx] : ",
				     (defined($ary->[$idx])
					 ? "'$ary->[$idx]'"
					 : "undef"),
				     "\n";
			   }
		       }
		   }

	       would print out:

		   $-{A}[0] : '1'
		   $-{A}[1] : '3'
		   $-{B}[0] : '2'
		   $-{B}[1] : '4'

	       The keys of the "%-" hash correspond to all buffer names found in the regular
	       expression.

	       The behaviour of "%-" is implemented via the Tie::Hash::NamedCapture module.

	       Note: "%-" and "%+" are tied views into a common internal hash associated with the
	       last successful regular expression.  Therefore mixing iterative access to them via
	       "each" may have unpredictable results.  Likewise, if the last successful match
	       changes, then the results may be surprising.

	       This variable was added in Perl 5.10

	       This variable is read-only and dynamically-scoped.

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

	       This variable was added in Perl 5.005.

       ${^RE_DEBUG_FLAGS}
	       The current value of the regex debugging flags.	Set to 0 for no debug output even
	       when the "re 'debug'" module is loaded.	See re for details.

	       This variable was added in Perl 5.10.

       ${^RE_TRIE_MAXBUF}
	       Controls how certain regex optimisations are applied and how much memory they
	       utilize.  This value by default is 65536 which corresponds to a 512kB temporary
	       cache.  Set this to a higher value to trade memory for speed when matching large
	       alternations.  Set it to a lower value if you want the optimisations to be as
	       conservative of memory as possible but still occur, and set it to a negative value
	       to prevent the optimisation and conserve the most memory.  Under normal situations
	       this variable should be of no interest to you.

	       This variable was added in Perl 5.10.

   Variables related to filehandles
       Variables that depend on the currently selected filehandle may be set by calling an
       appropriate 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,

	   HANDLE->method(EXPR)

       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
       "autoflush()", 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..3 ){
	       $\ = "\r\n";
	       nasty_break();
	       print "$_";
	   }

	   sub nasty_break {
	       $\ = "\f";
	       # do something with $_
	   }

       You probably expect this code to print the equivalent of

	   "1\r\n2\r\n3\r\n"

       but instead you get:

	   "1\f2\f3\f"

       Why? Because "nasty_break()" modifies "$\" without localizing it first.	The value you set
       in  "nasty_break()" is still there when you return.  The fix is to add "local()" so the
       value doesn't leak out of "nasty_break()":

	   local $\ = "\f";

       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.

       $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
	       name.

       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.

       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.

       Handle->output_field_separator( EXPR )
       $OUTPUT_FIELD_SEPARATOR
       $OFS
       $,      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.

       HANDLE->input_line_number( EXPR )
       $INPUT_LINE_NUMBER
       $NR
       $.      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.

       HANDLE->input_record_separator( EXPR )
       $INPUT_RECORD_SEPARATOR
       $RS
       $/      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
	       consecutive 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.

		   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
	       file.

	       On VMS only, record reads bypass PerlIO layers and any associated buffering,so you
	       must not mix record and non-record reads on the same filehandle.  Record mode
	       mixes with line mode only when the same buffering layer is in use for both modes.

	       If you perform a record read on a FILE with an encoding layer such as
	       ":encoding(latin1)" or ":utf8", you may get an invalid string as a result, may
	       leave the FILE positioned between characters in the stream and may not be reading
	       the number of bytes from the underlying file that you specified.  This behaviour
	       may change without warning in a future version of perl.

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

	       Mnemonic: / delimits line boundaries when quoting poetry.

       Handle->output_record_separator( EXPR )
       $OUTPUT_RECORD_SEPARATOR
       $ORS
       $\      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.

       HANDLE->autoflush( EXPR )
       $OUTPUT_AUTOFLUSH
       $|      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
	       channel 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 perlfunc on how to select the output channel.  See also
	       IO::Handle.

	       Mnemonic: when you want your pipes to be piping hot.

       Variables related to formats

       The special variables for formats are a subset of those for filehandles.  See perlform for
       more information about Perl's formats.

       $ACCUMULATOR
       $^A     The current value of the "write()" accumulator for "format()" lines.  A format
	       contains "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 PICTURE,LIST" in perlfunc.

       HANDLE->format_formfeed(EXPR)
       $FORMAT_FORMFEED
       $^L     What formats output as a form feed.  The default is "\f".

       HANDLE->format_page_number(EXPR)
       $FORMAT_PAGE_NUMBER
       $%      The current page number of the currently selected output channel.

	       Mnemonic: "%" is page number in nroff.

       HANDLE->format_lines_left(EXPR)
       $FORMAT_LINES_LEFT
       $-      The number of lines left on the page of the currently selected output channel.

	       Mnemonic: lines_on_page - lines_printed.

       Handle->format_line_break_characters EXPR
       $FORMAT_LINE_BREAK_CHARACTERS
       $:      The current set of characters after which a string may be broken to fill
	       continuation fields (starting with "^") in a format.  The default is " \n-", to
	       break on a space, newline, or a hyphen.

	       Mnemonic: a "colon" in poetry is a part of a line.

       HANDLE->format_lines_per_page(EXPR)
       $FORMAT_LINES_PER_PAGE
       $=      The current page length (printable lines) of the currently selected output
	       channel.  The default is 60.

	       Mnemonic: = has horizontal lines.

       HANDLE->format_top_name(EXPR)
       $FORMAT_TOP_NAME
       $^      The name of the current top-of-page format for the currently selected output
	       channel.  The default is the name of the filehandle with "_TOP" appended.  For
	       example, the default format top name for the "STDOUT" filehandle is "STDOUT_TOP".

	       Mnemonic: points to top of page.

       HANDLE->format_name(EXPR)
       $FORMAT_NAME
       $~      The name of the current report format for the currently selected output channel.
	       The default format name is the same as the filehandle name.  For example, the
	       default format name for the "STDOUT" filehandle is just "STDOUT".

	       Mnemonic: brother to $^.

   Error Variables
       The variables $@, $!, $^E, and $? contain information about different types of error
       conditions 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
       expression, which uses a single-quoted string.  After execution of this statement, perl
       may have set all four special error variables:

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

       When perl executes the "eval()" expression, it translates the "open()", "<PIPE>", and
       "close" calls in the C run-time library and thence to the operating system kernel.  perl
       sets $! to the C library's "errno" if one of these calls fails.

       $@ 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.)

       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
       program'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 $?.

       ${^CHILD_ERROR_NATIVE}
	       The native status returned by the last pipe close, backtick ("``") command,
	       successful 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 module.

	       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.

	       This variable was added in Perl 5.8.9.

       $EXTENDED_OS_ERROR
       $^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.

	       This variable was added in Perl 5.003.

	       Mnemonic: Extra error explanation.

       $EXCEPTIONS_BEING_CAUGHT
       $^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.

	       This variable was added in Perl 5.004.

       $WARNING
       $^W     The current value of the warning switch, initially true if -w was used, false
	       otherwise, but directly modifiable.

	       See also warnings.

	       Mnemonic: related to the -w switch.

       ${^WARNING_BITS}
	       The current set of warning checks enabled by the "use warnings" pragma.	It has
	       the same scoping as the $^H and "%^H" variables.  The exact values are considered
	       internal to the warnings pragma and may change between versions of Perl.

	       This variable was added in Perl 5.6.

       $OS_ERROR
       $ERRNO
       $!      When referenced, $! retrieves the current value of the C "errno" integer variable.
	       If $! is assigned a numerical value, that value is stored in "errno".  When
	       referenced as a string, $! yields the system error string corresponding to
	       "errno".

	       Many system or library calls set "errno" if they fail, to indicate the cause of
	       failure.  They usually do not set "errno" to zero if they succeed.  This means
	       "errno", hence $!, 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,
		   # $! is meaningless.

	       Here, meaningless means that $! may be unrelated to the outcome of the "open()"
	       operator.  Assignment to $! is similarly ephemeral.  It can be used immediately
	       before invoking the "die()" operator, to set the exit value, or to inspect the
	       system error string corresponding to error n, or to restore $! to a meaningful
	       state.

	       Mnemonic: What just went bang?

       %OS_ERROR
       %ERRNO
       %!      Each element of "%!" has a true value only if $! is set to that value.  For
	       example, $!{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
	       equivalent: not all operating systems give that exact error, and certainly not all
	       languages).  To check if a particular key is meaningful on your system, use
	       "exists $!{the_key}"; for a list of legal keys, use "keys %!".  See Errno for more
	       information, and also see "$!".

	       This variable was added in Perl 5.005.

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

	       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
	       details.

	       Mnemonic: similar to sh and ksh.

       $EVAL_ERROR
       $@      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).

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

	       Mnemonic: Where was the syntax error "at"?

   Variables related to the interpreter state
       These variables provide information about the current interpreter state.

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

	       This variable was added in Perl 5.6.

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

	       Mnemonic: value of -D switch.

       ${^ENCODING}
	       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 discouraged.

	       This variable was added in Perl 5.8.2.

       ${^GLOBAL_PHASE}
	       The current phase of the perl interpreter.

	       Possible values are:

	       CONSTRUCT
		       The "PerlInterpreter*" is being constructed via "perl_construct".  This
		       value is mostly there for completeness and for use via the underlying C
		       variable "PL_phase".  It's not really possible for Perl code to be
		       executed unless construction of the interpreter is finished.

	       START   This is the global compile-time.  That includes, basically, every "BEGIN"
		       block executed directly or indirectly from during the compile-time of the
		       top-level program.

		       This phase is not called "BEGIN" to avoid confusion with "BEGIN"-blocks,
		       as those are executed during compile-time of any compilation unit, not
		       just the top-level program.  A new, localised compile-time entered at run-
		       time, for example by constructs as "eval "use SomeModule"" are not global
		       interpreter phases, and therefore aren't reflected by "${^GLOBAL_PHASE}".

	       CHECK   Execution of any "CHECK" blocks.

	       INIT    Similar to "CHECK", but for "INIT"-blocks, not "CHECK" blocks.

	       RUN     The main run-time, i.e. the execution of "PL_main_root".

	       END     Execution of any "END" blocks.

	       DESTRUCT
		       Global destruction.

	       Also note that there's no value for UNITCHECK-blocks.  That's because those are
	       run for each compilation unit individually, and therefore is not a global
	       interpreter phase.

	       Not every program has to go through each of the possible phases, but transition
	       from one phase to another can only happen in the order described in the above
	       list.

	       An example of all of the phases Perl code can see:

		   BEGIN { print "compile-time: ${^GLOBAL_PHASE}\n" }

		   INIT  { print "init-time: ${^GLOBAL_PHASE}\n" }

		   CHECK { print "check-time: ${^GLOBAL_PHASE}\n" }

		   {
		       package Print::Phase;

		       sub new {
			   my ($class, $time) = @_;
			   return bless \$time, $class;
		       }

		       sub DESTROY {
			   my $self = shift;
			   print "$$self: ${^GLOBAL_PHASE}\n";
		       }
		   }

		   print "run-time: ${^GLOBAL_PHASE}\n";

		   my $runtime = Print::Phase->new(
		       "lexical variables are garbage collected before END"
		   );

		   END	 { print "end-time: ${^GLOBAL_PHASE}\n" }

		   our $destruct = Print::Phase->new(
		       "package variables are garbage collected after END"
		   );

	       This will print out

		   compile-time: START
		   check-time: CHECK
		   init-time: INIT
		   run-time: RUN
		   lexical variables are garbage collected before END: RUN
		   end-time: END
		   package variables are garbage collected after END: DESTRUCT

	       This variable was added in Perl 5.14.0.

       $^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
	       compilation 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() }
		       bar->baz($boon);
		   }

	       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
	       compiled.  The new value of $^H will therefore be visible only while the body of
	       "foo()" is being compiled.

	       Substitution of "BEGIN { add_100() }" 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
		   }

	       This variable was added in Perl 5.003.

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

	       When putting items into "%^H", in order to avoid conflicting with other users of
	       the hash there is a convention regarding which keys to use.  A module should use
	       only keys that begin with the module's name (the name of its main package) and a
	       "/" character.  For example, a module "Foo::Bar" should use keys such as
	       "Foo::Bar/baz".

	       This variable was added in Perl 5.6.

       ${^OPEN}
	       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.

	       This variable was added in Perl 5.8.0.

       $PERLDB
       $^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
		     compiled.

	       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.

       ${^TAINT}
	       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).

	       This variable is read-only.

	       This variable was added in Perl 5.8.

       ${^UNICODE}
	       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 was added in Perl 5.8.2.

       ${^UTF8CACHE}
	       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 was added in Perl 5.8.9.

       ${^UTF8LOCALE}
	       This variable indicates whether a 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.

	       This variable was added in Perl 5.8.8.

   Deprecated and removed variables
       Deprecating a variable announces the intent of the perl maintainers to eventually remove
       the variable from the language.	It may still be available despite its status.  Using a
       deprecated variable triggers a warning.

       Once a variable is removed, its use triggers an error telling you the variable is
       unsupported.

       See perldiag for details about error messages.

       $OFMT
       $#      $# was a variable that could be used to format printed numbers.	After a
	       deprecation cycle, its magic was removed in Perl 5.10 and using it now triggers a
	       warning: "$# is no longer supported".

	       This is not the sigil you use in front of an array name to get the last index,
	       like $#array.  That's still how you get the last index of an array in Perl.  The
	       two have nothing to do with each other.

	       Deprecated in Perl 5.

	       Removed in Perl 5.10.

       $*      $* was a variable that you could use to enable multiline matching.  After a
	       deprecation cycle, its magic was removed in Perl 5.10.  Using it now triggers a
	       warning: "$* is no longer supported".  You should use the "/s" and "/m" regexp
	       modifiers instead.

	       Deprecated in Perl 5.

	       Removed in Perl 5.10.

       $ARRAY_BASE
       $[      This variable stores the index of the first element in an array, and of the first
	       character in a substring.  The 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.

	       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.

	       Prior to Perl 5.10, assignment to $[ could be seen from outer lexical scopes in
	       the same file, unlike other compile-time directives (such as strict).  Using
	       local() on it would bind its value strictly to a lexical block.	Now it is always
	       lexically scoped.

	       As of Perl 5.16, it is implemented by the arybase module.  See arybase for more
	       details on its behaviour.

	       Under "use v5.16", or "no feature "array_base"", $[ no longer has any effect, and
	       always contains 0.  Assigning 0 to it is permitted, but any other value will
	       produce an error.

	       Mnemonic: [ begins subscripts.

	       Deprecated in Perl 5.12.

       $OLD_PERL_VERSION
       $]      See "$^V" for a more modern representation of the Perl version that allows
	       accurate string comparisons.

	       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:

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

	       The floating point representation can sometimes lead to inaccurate numeric
	       comparisons.

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

	       Mnemonic: Is this version of perl in the right bracket?

perl v5.16.3				    2013-03-04				       PERLVAR(1)
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