Anonymous Monk,
The question about how perl uses memory and if it returns it to the OS comes up from time to time. The answer is always - "it depends". See this node in which I have a number of links which might help.

As has already been answered by others, when perl terminates, the memory is returned back to the OS. You haven't mentioned what your application is, but if it is mod_perl for instance it might be a horse of a different color. Additionally, intentional circular references are an oddity for sure - what are you using them for? We can probably suggest a better way.