My point is that if you use anything but the publicly defined interface, you take your life into your own hands. Make the interface to each class clean and you don't have to worry about breaking people that don't use it. I don't feel bad if I break the code of someone who relied on a non-public interface to one of my classes. Some people don't understand that you don't need things like protected or private.

As an aside, the openness or closedness of the source has little to do with this conversation. There are plenty of open source Java programs that must implement the class control mechanisms mentioned above because that's the way that that language works. In its current incarnation, Perl has no such notions.