perlvar.pod

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=item ${^UNICODE}

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

=item ${^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.

=item ${^UTF8LOCALE}

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

=item $PERL_VERSION

=item $^V
X<$^V> X<$PERL_VERSION>

The revision, version, and subversion of the Perl interpreter, represented
as a C<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.  (Mnemonic: use ^V for Version
Control.)  Example:

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

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

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

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

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

=item $WARNING

=item $^W
X<$^W> X<$WARNING>

The current value of the warning switch, initially true if B<-w>
was used, false otherwise, but directly modifiable.  (Mnemonic:
related to the B<-w> switch.)  See also L<warnings>.

=item ${^WARNING_BITS}

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

=item ${^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determined and file attributes may be out of date if additional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 F<sitecustomize.pl> file to
configure the local Perl installation to use "sloppy" stat() by
default.  See L<perlrun> for more information about site
customization.

=item $EXECUTABLE_NAME

=item $^X
X<$^X> X<$EXECUTABLE_NAME>

The name used to execute the current copy of Perl, from C's
C<argv[0]> or (where supported) F</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;
  $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;
  $secure_perl_path = $Config{perlpath};
  if ($^O ne 'VMS')
     {$secure_perl_path .= $Config{_exe}
          unless $secure_perl_path =~ m/$Config{_exe}$/i;}

=item ARGV
X<ARGV>

The special filehandle that iterates over command-line filenames in
C<@ARGV>. Usually written as the null filehandle in the angle operator
C<< <> >>. Note that currently C<ARGV> only has its magical effect
within the C<< <> >> operator; elsewhere it is just a plain filehandle
corresponding to the last file opened by C<< <> >>. In particular,
passing C<\*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 C<@ARGV>.

=item $ARGV
X<$ARGV>

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

=item @ARGV
X<@ARGV>

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

=item ARGVOUT
X<ARGVOUT>

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

=item @F
X<@F>

The array @F contains the fields of each line read in when autosplit
mode is turned on.  See L<perlrun> for the B<-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 C<strict 'vars'>.

=item @INC
X<@INC>

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

=item @ARG

=item @_
X<@_> X<@ARG>

Within a subroutine the array @_ contains the parameters passed to that
subroutine.  See L<perlsub>.

=item %INC
X<%INC>

The hash %INC contains entries for each filename included via the
C<do>, C<require>, or C<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 C<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
L<perlfunc/require> 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.

=item %ENV

=item $ENV{expr}
X<%ENV>

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

=item %SIG

=item $SIG{expr}
X<%SIG>

The hash C<%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 C<'IGNORE'> usually has the effect of ignoring the
signal, except for the C<CHLD> signal.  See L<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 L<perlipc> for more information.

Certain internal hooks can be also set using the %SIG hash.  The
routine indicated by C<$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 C<__WARN__> hook causes the ordinary printing
of warnings to C<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 C<'IGNORE'> hook is not supported by C<__WARN__>, you can
disable warnings using the empty subroutine:

    local $SIG{__WARN__} = sub {};

The routine indicated by C<$SIG{__DIE__}> is called when a fatal exception
is about to be thrown.  The error message is passed as the first
argument.  When a C<__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 C<goto>, a loop exit, or a C<die()>.
The C<__DIE__> handler is explicitly disabled during the call, so that you
can die from a C<__DIE__> handler.  Similarly for C<__WARN__>.

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

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

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

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

See L<perlfunc/die>, L<perlfunc/warn>, L<perlfunc/eval>, and
L<warnings> for additional information.

=back

=head2 Error Indicators
X<error> X<exception>

The variables C<$@>, C<$!>, C<$^E>, and C<$?> 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:

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

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

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

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

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

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

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

=head2 Technical Note on the Syntax of Variable Names

Variable names in Perl can have several formats.  Usually, they
must begin with a letter or underscore, in which case they can be
arbitrarily long (up to an internal limit of 251 characters) and
may contain letters, digits, underscores, or the special sequence
C<::> or C<'>.  In this case, the part before the last C<::> or
C<'> is taken to be a I<package qualifier>; see L<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 C<^X> (caret C<X>) to mean the control-C<X>
character.  For example, the notation C<$^W> (dollar-sign caret
C<W>) is the scalar variable whose name is the single character
control-C<W>.  This is better than typing a literal control-C<W>
into your program.

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

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

	ENV		STDIN
	INC		STDOUT
	ARGV		STDERR
	ARGVOUT		_
	SIG

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

=head1 BUGS

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

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