I think undef will be fine if you are dumping the hash or array.

Also how many elements are you talking about? I have to ask because one of my partners was concerned about the same thing and he had 100 elements? ;)

Many machines these days have tons of ram and it quickly recovers stuff once a job completes. So it is good to be clean but don't get too freeked out unless of course you have gigantic arrays and hashes.

Just my .02