Reference Operations

If $x and $y are references, you can compare them with ==:

        $x == $y

This is true if and only if $x and $y refer to the same thing.
You can use this to recognize a reference that you have seen before:

        sub index_of_object {
          my ($obj, @o_list) = @_;
          my $i;
          for ($i=0; $i < @o_list; $i++) {
            return $i if $o_list[$i] == $obj;
          }
          return;
        }

Or use it as an optimization:

        sub obj_eq {
          my ($a, $b) = @_;
          return 1 if $a == $b;
          deep_compare($a, $b);
        }