perlvar.pod

source of perl for linux application,

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
F<INSTALL> file in the Perl distribution for information on how to
enable this option.  To discourage casual use of this advanced
feature, there is no L<English|English> long name for this variable.

=item $OSNAME

=item $^O

The name of the operating system under which this copy of Perl was
built, as determined during the configuration process.  The value
is identical to C<$Config{'osname'}>.  See also L<Config> and the 
B<-V> command-line switch documented in L<perlrun>.

=item $PERLDB

=item $^P

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

=over 6

=item 0x01

Debug subroutine enter/exit.

=item 0x02

Line-by-line debugging.

=item 0x04

Switch off optimizations.

=item 0x08

Preserve more data for future interactive inspections.

=item 0x10

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

=item 0x20

Start with single-step on.

=item 0x40

Use subroutine address instead of name when reporting.

=item 0x80

Report C<goto &subroutine> as well.

=item 0x100

Provide informative "file" names for evals based on the place they were compiled.

=item 0x200

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

=back

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

=item $LAST_REGEXP_CODE_RESULT

=item $^R

The result of evaluation of the last successful C<(?{ code })>
regular expression assertion (see L<perlre>).  May be written to.

=item $EXCEPTIONS_BEING_CAUGHT

=item $^S

Current state of the interpreter.  Undefined if parsing of the current
module/eval is not finished (may happen in $SIG{__DIE__} and
$SIG{__WARN__} handlers).  True if inside an eval(), otherwise false.

=item $BASETIME

=item $^T

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

=item $PERL_VERSION

=item $^V

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

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

    warn "No \"our\" declarations!\n" if $^V and $^V lt v5.6.0;

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

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 ${^WIDE_SYSTEM_CALLS}

Global flag that enables system calls made by Perl to use wide character
APIs native to the system, if available.  This is currently only implemented
on the Windows platform.

This can also be enabled from the command line using the C<-C> switch.

The initial value is typically C<0> for compatibility with Perl versions
earlier than 5.6, but may be automatically set to C<1> by Perl if the system
provides a user-settable default (e.g., C<$ENV{LC_CTYPE}>).

The C<bytes> pragma always overrides the effect of this flag in the current
lexical scope.  See L<bytes>.

=item $EXECUTABLE_NAME

=item $^X

The name that the Perl binary itself was executed as, from C's C<argv[0]>.
This may not be a full pathname, nor even necessarily in your path.

=item $ARGV

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

=item @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 @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.  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;

=item @_

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

=item %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.

=item %ENV

=item $ENV{expr}

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}

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 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.  If
your system has the SA_RESTART flag it is used when signals handlers are
installed.  This means that system calls for which restarting is supported
continue rather than returning when a signal arrives.  If you want your
system calls to be interrupted by signal delivery then do something like
this:

    use POSIX ':signal_h';

    my $alarm = 0;
    sigaction SIGALRM, new POSIX::SigAction sub { $alarm = 1 }
    	or die "Error setting SIGALRM handler: $!\n";

See L<POSIX>.

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 __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;

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 __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 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 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

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 PIPE, "/cdrom/install |";
	@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>.  A few
other names are also exempt:

	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.perl.com/CPAN/modules/by-module/Devel/)
for more information.

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.