...
  print "\nbefore deallocating the huge hash\n" . qx {ps -o rss $$};
  undef %x;
  print "\nafter deallocating the huge hash\n" . qx {ps -o rss $$};
  ...

##</code><code>##

Initial size:
  RSS
 1684

after allocating a huge hash:
  RSS
 9088

before deallocating the huge hash
  RSS
12532

after deallocating the huge hash
  RSS
12020

after the huge hash goes out of scope
  RSS
12020