This is probably old hat for those selected few among you that follow P5P, but yesterday, Anno Siegel posted an interesting article on comp.lang.perl.misc for the rest of us, entitled "Perl core support for inside-out classes". An excerpt:
As of v5.10 Perl will provide support for inside-out classes in form of a new core module Hash::Util::FieldHash. It is already available in the current bleadperl.

Inside-out classes store object data in hashes keyed by the object. That way each field of a class is realized as one lexical hash which may thus be called a field hash.

You can read the whole post on Google Groups, here.