perlvar man page

perlvar — Perl predefined variables


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. A Unicode letter that is not ASCII is not considered to be a letter unless "use utf8" is in effect, and somewhat more complicated rules apply; see "Identifier parsing" in perldata for details.

Perl variable names may also be a sequence of digits, a single punctuation character, or the two-character sequence: "^" (caret or CIRCUMFLEX ACCENT) followed by any one of the characters "[][A-Z^_?\]". 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.

Since Perl v5.6.0, Perl variable names may also be alphanumeric strings preceded by a caret. These must all be written in the form "${^Foo}"; the braces are not optional. "${^Foo}" denotes the scalar variable whose name is considered to be 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 "^_" (caret-underscore). No 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 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:


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. For more info, please see English.

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


The default input and pattern-searching space. The following pairs are equivalent:

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 use $_ as a default argument:

abs, alarm, chomp, chop, chr, chroot, cos, defined, eval, evalbytes, exp, fc, glob, hex, int, lc, lcfirst, length, log, lstat, mkdir, oct, ord, pos, print, printf, quotemeta, readlink, readpipe, ref, require, reverse (in scalar context only), rmdir, say, sin, split (for 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 supplied.
The implicit iterator variable in the "grep()" and "map()" functions.
The implicit variable of "given()".
The default place to put the next value or input record when a "<FH>", "readline", "readdir" or "each" operation's result is tested by itself as the sole criterion of a "while" test. Outside a "while" test, this will not happen.

$_ is by default a global variable. However, as of perl v5.10.0, you can 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. Though this seemed like a good idea at the time it was introduced, lexical $_ actually causes more problems than it solves. If you call a function that expects to be passed information via $_, it may or may not work, depending on how the function is written, there not being any easy way to solve this. Just avoid lexical $_, unless you are feeling particularly masochistic. For this reason lexical $_ is still experimental and will produce a warning unless warnings have been disabled. As with other experimental features, the behavior of lexical $_ is subject to change without notice, including change into a fatal error.

Mnemonic: underline is understood in certain operations.

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

See perlsub.

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.

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 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.
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 v5.14.0 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.
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.
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.
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.

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.


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


it really means

$foo{join($;, $x, $y, $z)}

But don't put

@foo{$x,$y,$z}      # 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 $;.

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

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

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.

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

As of v5.18.0, both keys and values stored in %ENV are stringified.

my $foo = 1;
$ENV{'bar'} = \$foo;
if( ref $ENV{'bar'} ) {
    say "Pre 5.18.0 Behaviour";
} else {
    say "Post 5.18.0 Behaviour";

Previously, only child processes received stringified values:

my $foo = 1;
$ENV{'bar'} = \$foo;
# Always printed 'non ref'
system($^X, '-e',
       q/print ( ref $ENV{'bar'}  ? 'ref' : 'non ref' ) /);

This happens because you can't really share arbitrary data structures with foreign processes.


The revision, version, and subversion of the Perl interpreter, represented as a decimal of the form 5.XXXYYY, where XXX is the version / 1e3 and YYY is the subversion / 1e6. For example, Perl v5.10.1 would be "5.010001".

This variable can be used to determine whether the Perl interpreter executing a script is in the right range of versions:

warn "No PerlIO!\n" if $] lt '5.008';

When comparing $], string comparison operators are highly recommended. The inherent limitations of binary floating point representation can sometimes lead to incorrect comparisons for some numbers on some architectures.

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

See "$^V" for a representation of the Perl version as a version object, which allows more flexible string comparisons.

The main advantage of $] over $^V is that it works the same on any version of Perl. The disadvantages are that it can't easily be compared to versions in other formats (e.g. literal v-strings, "v1.2.3" or version objects) and numeric comparisons can occasionally fail; it's good for string literal version checks and bad for comparing to a variable that hasn't been sanity-checked.

The $OLD_PERL_VERSION form was added in Perl v5.20.0 for historical reasons but its use is discouraged. (If your reason to use $] is to run code on old perls then referring to it as $OLD_PERL_VERSION would be self-defeating.)

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

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()".
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'".

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.

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.
The current value of the inplace-edit extension. Use "undef" to disable inplace editing.

Mnemonic: value of -i switch.

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.

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.

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 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 v5.8.0 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.

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.

The revision, version, and subversion of the Perl interpreter, represented as a version object.

This variable first appeared in perl v5.6.0; earlier versions of perl will see an undefined value. Before perl v5.10.0 $^V was represented as a v-string rather than a version object.

$^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

While version objects overload stringification, to portably convert $^V into its string representation, use "sprintf()"'s "%vd" conversion, which works for both v-strings or version objects:

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 a decimal representation of the Perl version.

The main advantage of $^V over $] is that, for Perl v5.10.0 or later, it overloads operators, allowing easy comparison against other version representations (e.g. decimal, literal v-string, "v1.2.3", or objects). The disadvantage is that prior to v5.10.0, it was only a literal v-string, which can't be easily printed or compared.

Mnemonic: use ^V for a version object.

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 v5.10.0.

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;

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 a 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 an 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 $?.

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 v5.10.0.
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.

Current state of the interpreter.

$^S         State
---------   -------------------------------------
undef       Parsing module, eval, or main program
true (1)    Executing an eval
false (0)   Otherwise

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

The English name $EXCEPTIONS_BEING_CAUGHT is slightly misleading, because the "undef" value does not indicate whether exceptions are being caught, since compilation of the main program does not catch exceptions.

This variable was added in Perl 5.004.

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.
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 v5.6.0.

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?

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). The specific true value is not guaranteed, but in the past has generally been the numeric value of $!. 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.

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:

    $? = 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.

The Perl error from the last "eval" operator, i.e. the last exception that was caught. For "eval BLOCK", this is either a runtime error message or the string or reference "die" was called with. The "eval STRING" form also catches syntax errors and other compile time exceptions.

If no error occurs, "eval" sets $@ to the empty string.

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 error "at"?

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.

$# was a variable that could be used to format printed numbers. After a deprecation cycle, its magic was removed in Perl v5.10.0 and using it now triggers a warning: "$# 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 v5.10.0.
$* was a variable that you could use to enable multiline matching. After a deprecation cycle, its magic was removed in Perl v5.10.0. Using it now triggers a warning: "$* is no longer supported". You should use the "/s" and "/m" regexp modifiers instead.

Deprecated in Perl 5.

Removed in Perl v5.10.0.
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 v5.10.0, 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 v5.16.0, 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 v5.12.0.


Explore man page connections for perlvar(1).