A quick search of CPAN turned up Class::PseudoHash which explains:

Due to its impact on overall performance of ordinary hashes, pseudo-hashes are deprecated in perl v5.8, and will cease to exist in perl v5.10. By then, the fields pragma is supposed to change to use a different implementation.

Class::PseudoHash transparently supports PseudoHashes by simply adding two lines to your program. Now, if you're writting new code, you probably want to avoid using this.

I have no experience with this module.