Many OO CPAN modules are simplicity themselves to use and modify.
I don't agree. I think many OO modules are hard to modify - the latter meaning "subclass".
Here's coding problem.
Someone has written a BaseballPlayer class, to maintain statistics of players. Your task is to subclass it to create a BaseballPlayer::Pitcher class so the additional statistics for pitchers can be dealt with.
Where are you going to store your instance data?
If you answer "in the hash returned by the constructor of the BaseballPlayer class", or "in whatever object the superclass uses", or "that depends on how the BaseballPlayer class is implemented", you fail the test. And why do you fail? Because you are breaking the basic concept of object orientness: encapsulation. It shouldn't matter how the superclass is implemented. Unfortunally, Perl doesn't make it easy to use inheritance. It all works handy-dandy if you can enforce a certain style, implementation or module usage of the total inheritance tree, but that isn't usually the case. It wouldn't be code reuse if you had to write everything yourself, now would it?