This module provides the ability to use references as hash keys if you
first "tie" the hash variable to this module. Normally, only the keys
of the tied hash itself are preserved as references; to use references
as keys in hashes-of-hashes, use Tie::RefHash::Nestable, included as
part of Tie::RefHash.
Note, this module is part of all relevant perl dists, but various CPAN
packages are beginning to require bug fixes found in newer versions.