That's not quite right either. Most modern OS's have facilities for returning memory to the system (on UNIX, it's usually mmap(2)), but most software, including Perl, doesn't make good use of them. See, for example:

#!/usr/bin/perl

use constant SLEEP => 20;

warn "Starting\n";

my @arr = (1..1_000_000);
warn "Allocated\n";

sleep(SLEEP);

undef @arr;
warn "Freed.\n";

sleep(SLEEP);
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Clearly most of the memory allocated will be in contiguous blocks, and could be reclaimed by the OS, but if you watch top you'll see that almost none ever is.