See the current Perl documentation for Tie::RefHash.
Here is our local, out-dated (pre-5.6) version:

Tie::RefHash - use references as hash keys

require 5.004;
use Tie::RefHash;
tie HASHVARIABLE, 'Tie::RefHash', LIST;
untie HASHVARIABLE;

This module provides the ability to use references as hash keys if you first tie the hash variable to this module.
It is implemented using the standard perl
TIEHASH interface. Please see the tie entry in
perlfunc(1) and
perltie(1) for more information.
use Tie::RefHash;
tie %h, 'Tie::RefHash';
$a = [];
$b = {};
$c = \*main;
$d = \"gunk";
$e = sub { 'foo' };
%h = ($a => 1, $b => 2, $c => 3, $d => 4, $e => 5);
$a->[0] = 'foo';
$b->{foo} = 'bar';
for (keys %h) {
print ref($_), "\n";
}
Gurusamy Sarathy gsar@umich.edu
Version 1.2 15 Dec 1996
perl(1),
perlfunc(1),
perltie(1)