Programming Perl, 4th Edition

caller

caller EXPR
caller

This function returns information about the stack of current subroutine calls and such. Without an argument, it returns the package name, filename, and line number from which the currently executing subroutine was called:

($package, $filename, $line) = caller;

Here’s an example of an exceedingly picky function, making use of the special tokens _ _PACKAGE_ _ and _ _FILE_ _ described in Chapter 2:

sub careful {
    my ($package, $filename) = caller;
    unless ($package  eq _ _PACKAGE_ _ && $filename eq _ _FILE_ _) {
        die "You weren't supposed to call me, $package!";
    }
    say "called me safely";
}

sub safecall {
    careful();
}

When called with an argument, caller evaluates EXPR as the number of stack frames to go back before the current one. For example, an argument of 0 means the current stack frame, 1 means the caller, 2 means the caller’s caller, and so on. The function also reports additional information, as shown here:

my $i = 0;
while (my ($package, $filename, $line, $subroutine,
           $hasargs, $wantarray, $evaltext, $is_require,
           $hints, $bitmask, $hinthash) = caller($i++) )
{
    ...
}

If the frame is a subroutine call, $hasargs is true if it has its own @_ array (not one borrowed from its caller). Otherwise, $subroutine may be “(eval)” if the frame is not a subroutine call but an eval. If so, additional elements $evaltext and $is_require are set: $is_require is true if the frame is created by a require or use statement, and $evaltext contains the text of the eval EXPR statement. ...