To add a bit more to Discipulus' excellent point that memory is rarely released to the OS, note that Perl leaves that decision to the implementation of the C-level malloc function on each platform, as described in more detail at Re: Not able to release memory (malloc implementation).

I just added your question to the long list of references I keep on memory-related topics at:

In reply to Re: Memory efficient design by eyepopslikeamosquito
in thread Memory efficient design by harangzsolt33

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