Tim Bunce's excellent Profiling Memory Usage talk gives some insight into what's going on at the 09:20 minute mark:

sub foo {
   my $var = "X" x 10_000_000;
}
foo();
[download]

Tim explains that:

1. The buffer for the $var lexical is preserved (for next time you call the subroutine).
2. The compiler says hey "X" is a constant and 10_000_000 is a constant, so I'm gonna build you a 10 MB constant! And I'm going to keep it here to one side so when you call this subroutine again I can just copy it in for you! (aka Constant folding).

With recursion, things get much worse (sorry, I couldn't bring myself to watch that part :).

Though Tim's Devel::SizeMe module might be useful, AFAIK it is up for adoption and not being actively developed ATM.

I also keep a list of Memory Tools References (from this list, Mini-Tutorial: Perl's Memory Management by ikegami is definitely worth reading).