Excellent solution to the problem.

Another one could be to use the "decorator" design pattern instead of subclassing. Here is an interesting discussion of when and why to use this pattern:

http://www.stevenblack.com/Articles/PTN-Decorator.asp

(it's not Perl, but you get the idea)

From your description I would go with the subclass solution, but it's always good to know your options.

/J