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Perl Cookbook by Nathan Torkington, Tom Christiansen

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Determining the Caller’s Package

Problem

You need to find out the current or calling package.

Solution

To find the current package:

$this_pack = __PACKAGE__;

To find the caller’s package:

$that_pack = caller();

Discussion

The __PACKAGE__ symbol returns the package that the code is currently being compiled into. This doesn’t interpolate into double-quoted strings:

print "I am in package __PACKAGE__\n";              # WRONG!

                  I am in package __PACKAGE__

Needing to figure out the caller’s package arose more often in older code that received as input a string of code to be evaluated, or a filehandle, format, or directory handle name. Consider a call to a hypothetical runit function:

package Alpha;
runit('$line = <TEMP>');

package Beta;
sub runit {
    my $codestr = shift;
    eval $codestr;
    die if $@;
}

Because runit was compiled in a different package than was currently executing, when the eval runs, it will act as though it were passed $Beta::line and Beta::TEMP. The old workaround was to include your caller’s package first:

package Beta;
sub runit {
    my $codestr = shift;
    my $hispack = caller;
    eval "package $hispack; $codestr";
    die if $@;
}

That approach only works when $line is a global variable. If it’s lexical, that won’t help at all. Instead, arrange for runit to accept a reference to a subroutine:

package Alpha;
runit( sub { $line = <TEMP> } );

package Beta;
sub runit {
    my $coderef = shift;
    &$coderef();
}

This not only works with lexicals, it has the added benefit of checking the code’s syntax at compile time, ...

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