, Some of the reasons that most OS will not reclaim memory very much or at all until the process exits are:
Fragmentation -- are the small chunks of memory released valuable to other applications? If so does the OS have to restructure and virtualize the memory so that the free blocks are contiguous? At what cost?
Unix processes tend to live in two forms, quick running (ls, cat, du) and long running (syslog, inetd, oracle). For quick running processes the process will be over soon enough and the memory free issue is moot. For long running processes the process will generally need to alloc more memory anyways, may as well make it easy and quick by leaving it for the next alloc. This happens at little expense to your physical RAM as while that memory is freed the chunk can be swapped out and used by other applications.
You will see that on most modern unices that large block (one alloc, one free) tend to be reabsorbed by the OS -- although for instance on Solaris with an app compiled with gcc you will not see this. it all depends on the OS, the compiler and the libraries used. Not really a perl issue.