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.

[only used for perl-JE with openSUSE = 10.3]