perlvar.pod

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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, record reads are done with the equivalent of C<sysread>,
so it's best not to mix record and non-record reads on the same
file.  (This is unlikely to be a problem, because any file you'd
want to read in record mode is probably unusable in line mode.)
Non-VMS systems do normal I/O, so it's safe to mix record and
non-record reads of a file.

See also L<perlport/"Newlines">.  Also see C<$.>.

=item HANDLE->autoflush(EXPR)

=item $OUTPUT_AUTOFLUSH

=item $|
X<$|> X<autoflush> X<flush> X<$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; C<$|> 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 B<rsh> and want to see the output as it's
happening.  This has no effect on input buffering.  See L<perlfunc/getc>
for that.  (Mnemonic: when you want your pipes to be piping hot.)

=item IO::Handle->output_field_separator EXPR

=item $OUTPUT_FIELD_SEPARATOR

=item $OFS

=item $,
X<$,> X<$OFS> X<$OUTPUT_FIELD_SEPARATOR>

The output field separator for the print operator.  If defined, this
value is printed between each of print's arguments.  Default is C<undef>.
(Mnemonic: what is printed when there is a "," in your print statement.)

=item IO::Handle->output_record_separator EXPR

=item $OUTPUT_RECORD_SEPARATOR

=item $ORS

=item $\
X<$\> X<$ORS> X<$OUTPUT_RECORD_SEPARATOR>

The output record separator for the print operator.  If defined, this
value is printed after the last of print's arguments.  Default is C<undef>.
(Mnemonic: you set C<$\> instead of adding "\n" at the end of the print.
Also, it's just like C<$/>, but it's what you get "back" from Perl.)

=item $LIST_SEPARATOR

=item $"
X<$"> X<$LIST_SEPARATOR>

This is like C<$,> except that it applies to array and slice values
interpolated into a double-quoted string (or similar interpreted
string).  Default is a space.  (Mnemonic: obvious, I think.)

=item $SUBSCRIPT_SEPARATOR

=item $SUBSEP

=item $;
X<$;> X<$SUBSEP> X<SUBSCRIPT_SEPARATOR>

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 B<awk>.  If your
keys contain binary data there might not be any safe value for C<$;>.
(Mnemonic: comma (the syntactic subscript separator) is a
semi-semicolon.  Yeah, I know, it's pretty lame, but C<$,> is already
taken for something more important.)

Consider using "real" multidimensional arrays as described
in L<perllol>.

=item HANDLE->format_page_number(EXPR)

=item $FORMAT_PAGE_NUMBER

=item $%
X<$%> X<$FORMAT_PAGE_NUMBER>

The current page number of the currently selected output channel.
Used with formats.
(Mnemonic: % is page number in B<nroff>.)

=item HANDLE->format_lines_per_page(EXPR)

=item $FORMAT_LINES_PER_PAGE

=item $=
X<$=> X<$FORMAT_LINES_PER_PAGE>

The current page length (printable lines) of the currently selected
output channel.  Default is 60.  
Used with formats.
(Mnemonic: = has horizontal lines.)

=item HANDLE->format_lines_left(EXPR)

=item $FORMAT_LINES_LEFT

=item $-
X<$-> X<$FORMAT_LINES_LEFT>

The number of lines left on the page of the currently selected output
channel.  
Used with formats.
(Mnemonic: lines_on_page - lines_printed.)

=item @LAST_MATCH_START

=item @-
X<@-> X<@LAST_MATCH_START>

$-[0] is the offset of the start of the last successful match.
C<$-[>I<n>C<]> is the offset of the start of the substring matched by
I<n>-th subpattern, or undef if the subpattern did not match.

Thus after a match against $_, $& coincides with C<substr $_, $-[0],
$+[0] - $-[0]>.  Similarly, $I<n> coincides with C<substr $_, $-[n],
$+[n] - $-[n]> if C<$-[n]> is defined, and $+ coincides with
C<substr $_, $-[$#-], $+[$#-] - $-[$#-]>.  One can use C<$#-> to find the last
matched subgroup in the last successful match.  Contrast with
C<$#+>, the number of subgroups in the regular expression.  Compare
with C<@+>.

This array holds the offsets of the beginnings of the last
successful submatches in the currently active dynamic scope.
C<$-[0]> is the offset into the string of the beginning of the
entire match.  The I<n>th element of this array holds the offset
of the I<n>th submatch, so C<$-[1]> is the offset where $1
begins, C<$-[2]> the offset where $2 begins, and so on.

After a match against some variable $var:

=over 5

=item C<$`> is the same as C<substr($var, 0, $-[0])>

=item C<$&> is the same as C<substr($var, $-[0], $+[0] - $-[0])>

=item C<$'> is the same as C<substr($var, $+[0])>

=item C<$1> is the same as C<substr($var, $-[1], $+[1] - $-[1])>  

=item C<$2> is the same as C<substr($var, $-[2], $+[2] - $-[2])>

=item C<$3> is the same as C<substr($var, $-[3], $+[3] - $-[3])>

=back

=item %-
X<%->

Similar to C<%+>, this variable allows access to the named capture buffers
in the last successful match in the currently active dynamic scope. To
each capture buffer 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 C<%-> hash correspond to all buffer names found in
the regular expression.

The behaviour of C<%-> is implemented via the
L<Tie::Hash::NamedCapture> module.

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

=item HANDLE->format_name(EXPR)

=item $FORMAT_NAME

=item $~
X<$~> X<$FORMAT_NAME>

The name of the current report format for the currently selected output
channel.  Default is the name of the filehandle.  (Mnemonic: brother to
C<$^>.)

=item HANDLE->format_top_name(EXPR)

=item $FORMAT_TOP_NAME

=item $^
X<$^> X<$FORMAT_TOP_NAME>

The name of the current top-of-page format for the currently selected
output channel.  Default is the name of the filehandle with _TOP
appended.  (Mnemonic: points to top of page.)

=item IO::Handle->format_line_break_characters EXPR

=item $FORMAT_LINE_BREAK_CHARACTERS

=item $:
X<$:> X<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.  Default is
S<" \n-">, to break on whitespace or hyphens.  (Mnemonic: a "colon" in
poetry is a part of a line.)

=item IO::Handle->format_formfeed EXPR

=item $FORMAT_FORMFEED

=item $^L
X<$^L> X<$FORMAT_FORMFEED>

What formats output as a form feed.  Default is \f.

=item $ACCUMULATOR

=item $^A
X<$^A> X<$ACCUMULATOR>

The current value of the write() accumulator for format() lines.  A format
contains formline() calls that put their result into C<$^A>.  After
calling its format, write() prints out the contents of C<$^A> and empties.
So you never really see the contents of C<$^A> unless you call
formline() yourself and then look at it.  See L<perlform> and
L<perlfunc/formline()>.

=item $CHILD_ERROR

=item $?
X<$?> X<$CHILD_ERROR>

The status returned by the last pipe close, backtick (C<``>) 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 (C<<< $? >> 8 >>>), and
C<$? & 127> gives which signal, if any, the process died from, and
C<$? & 128> reports whether there was a core dump.  (Mnemonic:
similar to B<sh> and B<ksh>.)

Additionally, if the C<h_errno> variable is supported in C, its value
is returned via $? if any C<gethost*()> function fails.

If you have installed a signal handler for C<SIGCHLD>, the
value of C<$?> will usually be wrong outside that handler.

Inside an C<END> subroutine C<$?> contains the value that is going to be
given to C<exit()>.  You can modify C<$?> in an C<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 C<use vmsish 'status'> makes C<$?> reflect the
actual VMS exit status, instead of the default emulation of POSIX
status; see L<perlvms/$?> for details.

Also see L<Error Indicators>.

=item ${^CHILD_ERROR_NATIVE}
X<$^CHILD_ERROR_NATIVE>

The native status returned by the last pipe close, backtick (C<``>)
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 L<POSIX> module.

Under VMS this reflects the actual VMS exit status; i.e. it is the same
as $? when the pragma C<use vmsish 'status'> is in effect.

=item ${^ENCODING}
X<$^ENCODING>

The I<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 I<undef>.  The direct
manipulation of this variable is highly discouraged.

=item $OS_ERROR

=item $ERRNO

=item $!
X<$!> X<$ERRNO> X<$OS_ERROR>

If used numerically, yields the current value of the C C<errno>
variable, or in other words, if a system or library call fails, it
sets this variable.  This means that the value of C<$!> is meaningful
only I<immediately> after a B<failure>:

    if (open(FH, $filename)) {
	# Here $! is meaningless.
	...
    } else {
	# ONLY here is $! meaningful.
	...
	# Already here $! might be meaningless.
    }
    # Since here we might have either success or failure,
    # here $! is meaningless.

In the above I<meaningless> stands for anything: zero, non-zero,
C<undef>.  A successful system or library call does B<not> set
the variable to zero.

If used as a string, yields the corresponding system error string.
You can assign a number to C<$!> to set I<errno> if, for instance,
you want C<"$!"> to return the string for error I<n>, or you want
to set the exit value for the die() operator.  (Mnemonic: What just
went bang?)

Also see L<Error Indicators>.

=item %OS_ERROR

=item %ERRNO

=item %!
X<%!>

Each element of C<%!> has a true value only if C<$!> is set to that
value.  For example, C<$!{ENOENT}> is true if and only if the current
value of C<$!> is C<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
C<exists $!{the_key}>; for a list of legal keys, use C<keys %!>.
See L<Errno> for more information, and also see above for the
validity of C<$!>.

=item $EXTENDED_OS_ERROR

=item $^E
X<$^E> X<$EXTENDED_OS_ERROR>

Error information specific to the current operating system.  At
the moment, this differs from C<$!> under only VMS, OS/2, and Win32
(and for MacPerl).  On all other platforms, C<$^E> is always just
the same as C<$!>.

Under VMS, C<$^E> provides the VMS status value from the last
system error.  This is more specific information about the last
system error than that provided by C<$!>.  This is particularly
important when C<$!> is set to B<EVMSERR>.

Under OS/2, C<$^E> is set to the error code of the last call to
OS/2 API either via CRT, or directly from perl.

Under Win32, C<$^E> always returns the last error information
reported by the Win32 call C<GetLastError()> which describes
the last error from within the Win32 API.  Most Win32-specific
code will report errors via C<$^E>.  ANSI C and Unix-like calls
set C<errno> and so most portable Perl code will report errors
via C<$!>. 

Caveats mentioned in the description of C<$!> generally apply to
C<$^E>, also.  (Mnemonic: Extra error explanation.)

Also see L<Error Indicators>.

=item $EVAL_ERROR

=item $@
X<$@> X<$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