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.

It is implemented using the standard perl TIEHASH interface. Please
see the tie entry in perlfunc(1) and perltie(1) for more information.

The Nestable version works by looking for hash references being
stored and converting them to tied hashes so that they too can have
references as keys. This will happen without warning whenever you
store a reference to one of your own hashes in the tied hash.