Interestingly, my copy of Perl seems to return about half of the memory when you undef a variable, at least on Linux: 
sub showmem
{
  system("cat /proc/$$/status |grep '^Vm' |sed -e 's/\$/ ($_[0])/'");
  print "\n"
}

showmem("start");

my $x = "A" x 10_000_000;
showmem("allocated");

undef $x;
showmem("freed");

[download]
produces: 
VmSize:	    2808 kB (start)
VmLck:	       0 kB (start)
VmRSS:	    1144 kB (start)
VmData:	     240 kB (start)
VmStk:	      28 kB (start)
VmExe:	     684 kB (start)
VmLib:	    1528 kB (start)

VmSize:	   22348 kB (allocated)
VmLck:	       0 kB (allocated)
VmRSS:	   20700 kB (allocated)
VmData:	   19780 kB (allocated)
VmStk:	      28 kB (allocated)
VmExe:	     684 kB (allocated)
VmLib:	    1528 kB (allocated)

VmSize:	   12580 kB (freed)
VmLck:	       0 kB (freed)
VmRSS:	   10936 kB (freed)
VmData:	   10012 kB (freed)
VmStk:	      28 kB (freed)
VmExe:	     684 kB (freed)
VmLib:	    1528 kB (freed)

In reply to Re: Re: Memory usage breakup by sgifford
in thread Memory usage breakup by eXile

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