At some point, an inherited class has to know a little more about the internals of its parent than other code. This goes for any language, not just Perl. Not necessarily all the details, just more than normal.

Not in Smalltalk. You inherit from classes all the time without knowing their internals; only what messages they respond to. In fact, *every* object inherits--at some point--from "Object", the root of the inheritance hierarchy. I honestly have no idea how Object's methods are implemented internally by the Smalltalk distro I'm using right now (Squeak), but I know what messages I can send to objects derived from it and how they'll respond to those messages, and that's good enough.

It's not the fact that Perl doesn't prevent you from fiddling with the internals that rubs me the wrong way, it's the fact that you really can't do anything without fiddling.

I'm not trying to bash Perl and I don't think OO is the be-all end-all, but this is one area where, unfortunately, Perl is exceptionally weak. But that's ok, because it's good at everything else :)