I run into that problem alot doing Tk programming, since all Tk widgets are hashes of one sort or another. The undef, delete, or destroying of these hashes does not free the memory, and if you design that way, you will get applications which just keep growing in size. It's not too noticeable on one-shot quick GUIs, but on long-running programs, it can really become noticable.

In Tk, the only solution is to make your objects only ONCE and then reuse them. The previously mentioned solution about making the hash a global, then emptying it and reusing it, sounds about the right approach to me. Also watch out for auto-vivication of hashes, which causes them to "leak", read Autovivification with hash of hashes for example. It can be hard to effectively delete or undef an hash.