The Tie::RefHash module can be used to access hashes by reference. This is
useful when you index by object, for example.

The problem with Tie::RefHash, and cross indexing, is that sometimes the
index should not contain strong references to the objecs. Tie::RefHash's
internal structures contain strong references to the key, and provide no
convenient means to make those references weak.

This subclass of Tie::RefHash has weak keys, instead of strong ones. The
values are left unaltered, and you'll have to make sure there are no strong
references there yourself.