Should be obvious enough... Emptying or undef'ing a hash might or might not set the reference-count of the objects in question to zero. And even if it does, garbage collection might not occur immediately. So the statement is quite pragmatically correct: you can zap a hash, check the memory allocation immediately thereafter, and find that nothing at all has changed. Yet. If the OP finds that memory is not being released, odds are quite good that there are circular references going on here which haven't been weakened.
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in thread Memory issue with Hash