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, which is a major win.

If all that's being passed in is a filehandle, it's more portable to use the Symbol::qualify function. This function takes a name and package to qualify the name into. If the name needs qualification, it fixes it; otherwise, it's left alone. But that's considerably less efficient than a * prototype.

Here's an example that reads and returns n lines from a filehandle. The function qualifies the handle before working with it.

open (FH, "< /etc/termcap") or die "can't open /etc/termcap: $!"; ($a, $b, $c) = nreadline(3, 'FH'); use Symbol (); use Carp; sub nreadline {
 my ($count, $handle) = @_; my(@retlist,$line); croak "count must be > 0" unless $count > 0; $handle = Symbol::qualify($handle, (caller())[0]); croak "need open filehandle" unless defined fileno($handle); push(@retlist, $line) while defined($line = <$handle>) && $count--;
return @retlist;
}

If everyone who called your nreadline function passed in the filehandle as a typeglob *FH, as a glob reference \*FH, or using FileHandle or IO::Handle objects, you wouldn't need to do this. It's only the possibility of a bare "FH" that requires qualification.