FYI, the hash locking feature only started in 5.8.0. From the docs.

Restricted hashes

5.8.0 introduces the ability to restrict a hash to a certain set of keys. No keys outside of this set can be added. It also introduces the ability to lock an individual key so it cannot be deleted and the value cannot be changed.

This is intended to largely replace the deprecated pseudo-hashes.

And hooray for removing pseudohashes! I have the notes from Damian Conway's Advanced OO Perl talk (Boston 2001) that talks a bit about pseudohashes. They were a cool idea, but the implementation was very flawed. The more you read about them, the less interested you became.