I've encountered this before. I have a solution, but it is far from optimal. I ended up trading memory for CPU utilization. In my instance, this tradeoff worked well, but based on how you're using and accessing the data it may not be apropriate to you. With that disclaimer out of the way:

What I did was searlize all the objects in the hash. When I needed to access them I re-inflated them into full-fledged objects. Depending on your data you could even use on-disk storage (like a tied dbm file, or even a real DB) for the objects; only pulling into memory the ones you need. When I used this technique I did not go as far as to use on-disk storage for the data elements, but by just searilizing them I cut down my memory usage by nearly %75; of course, it took about twice as long to run. I used Storable, but there are several other good perl modules out there that can searilize your objects.