perlvar.1

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By default, running out of memory is an untrappable, fatal error.
However, if suitably built, Perl can use the contents of \f(CW$^M\fR
as an emergency memory pool after \fIdie()\fRing.  Suppose that your Perl
were compiled with \f(CW\*(C`\-DPERL_EMERGENCY_SBRK\*(C'\fR and used Perl's malloc.
Then
.Sp
.Vb 1
\&    $^M = \*(Aqa\*(Aq x (1 << 16);
.Ve
.Sp
would allocate a 64K buffer for use in an emergency.  See the
\&\fI\s-1INSTALL\s0\fR file in the Perl distribution for information on how to
add custom C compilation flags when compiling perl.  To discourage casual
use of this advanced feature, there is no English long name for
this variable.
.ie n .IP "$OSNAME" 8
.el .IP "\f(CW$OSNAME\fR" 8
.IX Item "$OSNAME"
.PD 0
.IP "$^O" 8
.IX Xref "$^O $OSNAME"
.IX Item "$^O"
.PD
The name of the operating system under which this copy of Perl was
built, as determined during the configuration process.  The value
is identical to \f(CW$Config{\*(Aqosname\*(Aq}\fR.  See also Config and the 
\&\fB\-V\fR command-line switch documented in perlrun.
.Sp
In Windows platforms, $^O is not very helpful: since it is always
\&\f(CW\*(C`MSWin32\*(C'\fR, it doesn't tell the difference between
95/98/ME/NT/2000/XP/CE/.NET.  Use \fIWin32::GetOSName()\fR or
\&\fIWin32::GetOSVersion()\fR (see Win32 and perlport) to distinguish
between the variants.
.IP "${^OPEN}" 8
.IX Item "${^OPEN}"
An internal variable used by PerlIO.  A string in two parts, separated
by a \f(CW\*(C`\e0\*(C'\fR byte, the first part describes the input layers, the second
part describes the output layers.
.ie n .IP "$PERLDB" 8
.el .IP "\f(CW$PERLDB\fR" 8
.IX Item "$PERLDB"
.PD 0
.IP "$^P" 8
.IX Xref "$^P $PERLDB"
.IX Item "$^P"
.PD
The internal variable for debugging support.  The meanings of the
various bits are subject to change, but currently indicate:
.RS 8
.IP "0x01" 6
.IX Item "0x01"
Debug subroutine enter/exit.
.IP "0x02" 6
.IX Item "0x02"
Line-by-line debugging.
.IP "0x04" 6
.IX Item "0x04"
Switch off optimizations.
.IP "0x08" 6
.IX Item "0x08"
Preserve more data for future interactive inspections.
.IP "0x10" 6
.IX Item "0x10"
Keep info about source lines on which a subroutine is defined.
.IP "0x20" 6
.IX Item "0x20"
Start with single-step on.
.IP "0x40" 6
.IX Item "0x40"
Use subroutine address instead of name when reporting.
.IP "0x80" 6
.IX Item "0x80"
Report \f(CW\*(C`goto &subroutine\*(C'\fR as well.
.IP "0x100" 6
.IX Item "0x100"
Provide informative \*(L"file\*(R" names for evals based on the place they were compiled.
.IP "0x200" 6
.IX Item "0x200"
Provide informative names to anonymous subroutines based on the place they
were compiled.
.IP "0x400" 6
.IX Item "0x400"
Debug assertion subroutines enter/exit.
.RE
.RS 8
.Sp
Some bits may be relevant at compile-time only, some at
run-time only.  This is a new mechanism and the details may change.
.RE
.ie n .IP "$LAST_REGEXP_CODE_RESULT" 8
.el .IP "\f(CW$LAST_REGEXP_CODE_RESULT\fR" 8
.IX Item "$LAST_REGEXP_CODE_RESULT"
.PD 0
.IP "$^R" 8
.IX Xref "$^R $LAST_REGEXP_CODE_RESULT"
.IX Item "$^R"
.PD
The result of evaluation of the last successful \f(CW\*(C`(?{ code })\*(C'\fR
regular expression assertion (see perlre).  May be written to.
.ie n .IP "$EXCEPTIONS_BEING_CAUGHT" 8
.el .IP "\f(CW$EXCEPTIONS_BEING_CAUGHT\fR" 8
.IX Item "$EXCEPTIONS_BEING_CAUGHT"
.PD 0
.IP "$^S" 8
.IX Xref "$^S $EXCEPTIONS_BEING_CAUGHT"
.IX Item "$^S"
.PD
Current state of the interpreter.
.Sp
.Vb 5
\&    $^S         State
\&    \-\-\-\-\-\-\-\-\-   \-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-\-
\&    undef       Parsing module/eval
\&    true (1)    Executing an eval
\&    false (0)   Otherwise
.Ve
.Sp
The first state may happen in \f(CW$SIG\fR{_\|_DIE_\|_} and \f(CW$SIG\fR{_\|_WARN_\|_} handlers.
.ie n .IP "$BASETIME" 8
.el .IP "\f(CW$BASETIME\fR" 8
.IX Item "$BASETIME"
.PD 0
.IP "$^T" 8
.IX Xref "$^T $BASETIME"
.IX Item "$^T"
.PD
The time at which the program began running, in seconds since the
epoch (beginning of 1970).  The values returned by the \fB\-M\fR, \fB\-A\fR,
and \fB\-C\fR filetests are based on this value.
.IP "${^TAINT}" 8
.IX Item "${^TAINT}"
Reflects if taint mode is on or off.  1 for on (the program was run with
\&\fB\-T\fR), 0 for off, \-1 when only taint warnings are enabled (i.e. with
\&\fB\-t\fR or \fB\-TU\fR).  This variable is read-only.
.IP "${^UNICODE}" 8
.IX Item "${^UNICODE}"
Reflects certain Unicode settings of Perl.  See perlrun
documentation for the \f(CW\*(C`\-C\*(C'\fR switch for more information about
the possible values. This variable is set during Perl startup
and is thereafter read-only.
.IP "${^UTF8CACHE}" 8
.IX Item "${^UTF8CACHE}"
This variable controls the state of the internal \s-1UTF\-8\s0 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.
.IP "${^UTF8LOCALE}" 8
.IX Item "${^UTF8LOCALE}"
This variable indicates whether an \s-1UTF\-8\s0 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 \f(CW\*(C`\-CL\*(C'\fR command-line
switch); see perlrun for more info on this.
.ie n .IP "$PERL_VERSION" 8
.el .IP "\f(CW$PERL_VERSION\fR" 8
.IX Item "$PERL_VERSION"
.PD 0
.IP "$^V" 8
.IX Xref "$^V $PERL_VERSION"
.IX Item "$^V"
.PD
The revision, version, and subversion of the Perl interpreter, represented
as a \f(CW\*(C`version\*(C'\fR object.
.Sp
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.
.Sp
$^V can be used to determine whether the Perl interpreter executing a
script is in the right range of versions.  (Mnemonic: use ^V for Version
Control.)  Example:
.Sp
.Vb 1
\&    warn "Hashes not randomized!\en" if !$^V or $^V lt v5.8.1
.Ve
.Sp
To convert \f(CW$^V\fR into its string representation use \fIsprintf()\fR's
\&\f(CW"%vd"\fR conversion:
.Sp
.Vb 1
\&    printf "version is v%vd\en", $^V;  # Perl\*(Aqs version
.Ve
.Sp
See the documentation of \f(CW\*(C`use VERSION\*(C'\fR and \f(CW\*(C`require VERSION\*(C'\fR
for a convenient way to fail if the running Perl interpreter is too old.
.Sp
See also \f(CW$]\fR for an older representation of the Perl version.
.ie n .IP "$WARNING" 8
.el .IP "\f(CW$WARNING\fR" 8
.IX Item "$WARNING"
.PD 0
.IP "$^W" 8
.IX Xref "$^W $WARNING"
.IX Item "$^W"
.PD
The current value of the warning switch, initially true if \fB\-w\fR
was used, false otherwise, but directly modifiable.  (Mnemonic:
related to the \fB\-w\fR switch.)  See also warnings.
.IP "${^WARNING_BITS}" 8
.IX Item "${^WARNING_BITS}"
The current set of warning checks enabled by the \f(CW\*(C`use warnings\*(C'\fR pragma.
See the documentation of \f(CW\*(C`warnings\*(C'\fR for more details.
.IP "${^WIN32_SLOPPY_STAT}" 8
.IX Item "${^WIN32_SLOPPY_STAT}"
If this variable is set to a true value, then \fIstat()\fR 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.
.Sp
This variable could be set in the \fIsitecustomize.pl\fR file to
configure the local Perl installation to use \*(L"sloppy\*(R" \fIstat()\fR by
default.  See perlrun for more information about site
customization.
.ie n .IP "$EXECUTABLE_NAME" 8
.el .IP "\f(CW$EXECUTABLE_NAME\fR" 8
.IX Item "$EXECUTABLE_NAME"
.PD 0
.IP "$^X" 8
.IX Xref "$^X $EXECUTABLE_NAME"
.IX Item "$^X"
.PD
The name used to execute the current copy of Perl, from C's
\&\f(CW\*(C`argv[0]\*(C'\fR or (where supported) \fI/proc/self/exe\fR.
.Sp
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 \s-1PATH\s0 environment variable, so there
is no guarantee that the value of $^X is in \s-1PATH\s0.  For \s-1VMS\s0, the
value may or may not include a version number.
.Sp
You usually can use the value of $^X to re-invoke an independent
copy of the same perl that is currently running, e.g.,
.Sp
.Vb 1
\&  @first_run = \`$^X \-le "print int rand 100 for 1..100"\`;
.Ve
.Sp
But recall that not all operating systems support forking or
capturing of the output of commands, so this complex statement
may not be portable.
.Sp
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:
.Sp
.Vb 6
\&  # Build up a set of file names (not command names).
\&  use Config;
\&  $this_perl = $^X;
\&  if ($^O ne \*(AqVMS\*(Aq)
\&     {$this_perl .= $Config{_exe}
\&          unless $this_perl =~ m/$Config{_exe}$/i;}
.Ve
.Sp
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.
.Sp
.Vb 5
\&  use Config;
\&  $secure_perl_path = $Config{perlpath};
\&  if ($^O ne \*(AqVMS\*(Aq)
\&     {$secure_perl_path .= $Config{_exe}
\&          unless $secure_perl_path =~ m/$Config{_exe}$/i;}
.Ve
.IP "\s-1ARGV\s0" 8
.IX Xref "ARGV"
.IX Item "ARGV"
The special filehandle that iterates over command-line filenames in
\&\f(CW@ARGV\fR. Usually written as the null filehandle in the angle operator
\&\f(CW\*(C`<>\*(C'\fR. Note that currently \f(CW\*(C`ARGV\*(C'\fR only has its magical effect
within the \f(CW\*(C`<>\*(C'\fR operator; elsewhere it is just a plain filehandle
corresponding to the last file opened by \f(CW\*(C`<>\*(C'\fR. In particular,
passing \f(CW\*(C`\e*ARGV\*(C'\fR as a parameter to a function that expects a filehandle
may not cause your function to automatically read the contents of all the
files in \f(CW@ARGV\fR.
.ie n .IP "$ARGV" 8
.el .IP "\f(CW$ARGV\fR" 8
.IX Xref "$ARGV"
.IX Item "$ARGV"
contains the name of the current file when reading from <>.
.ie n .IP "@ARGV" 8
.el .IP "\f(CW@ARGV\fR" 8
.IX Xref "@ARGV"
.IX Item "@ARGV"
The array \f(CW@ARGV\fR contains the command-line arguments intended for
the script.  \f(CW$#ARGV\fR is generally the number of arguments minus
one, because \f(CW$ARGV[0]\fR is the first argument, \fInot\fR the program's
command name itself.  See \f(CW$0\fR for the command name.
.IP "\s-1ARGVOUT\s0" 8
.IX Xref "ARGVOUT"
.IX Item "ARGVOUT"
The special filehandle that points to the currently open output file
when doing edit-in-place processing with \fB\-i\fR.  Useful when you have
to do a lot of inserting and don't want to keep modifying \f(CW$_\fR.  See
perlrun for the \fB\-i\fR switch.
.ie n .IP "@F" 8
.el .IP "\f(CW@F\fR" 8
.IX Xref "@F"
.IX Item "@F"
The array \f(CW@F\fR contains the fields of each line read in when autosplit
mode is turned on.  See perlrun for the \fB\-a\fR switch.  This array
is package-specific, and must be declared or given a full package name
if not in package main when running under \f(CW\*(C`strict \*(Aqvars\*(Aq\*(C'\fR.
.ie n .IP "@INC" 8
.el .IP "\f(CW@INC\fR" 8
.IX Xref "@INC"
.IX Item "@INC"
The array \f(CW@INC\fR contains the list of places that the \f(CW\*(C`do EXPR\*(C'\fR,
\&\f(CW\*(C`require\*(C'\fR, or \f(CW\*(C`use\*(C'\fR constructs look for their library files.  It
initially consists of the arguments to any \fB\-I\fR command-line
switches, followed by the default Perl library, probably
\&\fI/usr/local/lib/perl\fR, followed by \*(L".\*(R", to represent the current
directory.  (\*(L".\*(R" will not be appended if taint checks are enabled, either by
\&\f(CW\*(C`\-T\*(C'\fR or by \f(CW\*(C`\-t\*(C'\fR.)  If you need to modify this at runtime, you should use
the \f(CW\*(C`use lib\*(C'\fR pragma to get the machine-dependent library properly
loaded also:
.Sp
.Vb 2
\&    use lib \*(Aq/mypath/libdir/\*(Aq;
\&    use SomeMod;
.Ve
.Sp
You can also insert hooks into the file inclusion system by putting Perl
code directly into \f(CW@INC\fR.  Those hooks may be subroutine references, array
references or blessed objects.  See \*(L"require\*(R" in perlfunc for details.
.ie n .IP "@ARG" 8
.el .IP "\f(CW@ARG\fR" 8
.IX Item "@ARG"
.PD 0
.ie n .IP "@_" 8
.el .IP "\f(CW@_\fR" 8
.IX Xref "@_ @ARG"
.IX Item "@_"
.PD
Within a subroutine the array \f(CW@_\fR contains the parameters passed to that
subroutine.  See perlsub.
.ie n .IP "%INC" 8
.el .IP "\f(CW%INC\fR" 8
.IX Xref "%INC"