$n = minimum_key(\%h); # minimum_key left as an exercise
while (@n or ($n<maximum_key(\%h))) { # also maximum_key
   if not (@n) {
      push @n, keys_between(\%h, $n, $n+1);
      # Yep, keys_between is also left as an exercise.
      $n++;
   }
   my $thiselement = shift @n;
   process_element($thiselement);
}