Check out the question I wrote about Flyweight Objects and garbage collection to see an implementation of a "flyweight class". I got the idea from Damian Conway, who suggested it as a method of data hiding.

It does what you are doing -- stores the object in a list -- but instead of returning a blessed reference to the object, it returns a blessed index into the list. Add a DESTROY that undefs the entry in the list, and you get the behavior you want.