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.

Well, that depends on your definition of need.

While the _private_method naming convention is a nice signal to the reader that this particular subroutine is not part of the public API, it doesn't help with encapsulation. See Re^2: characterstics of private in perl and Re^4: Encapsulation through stringification - a variation on flyweight objects for some examples of where this sort of encapsulation problem can cause problems for people who are not deliberately poking at things they shouldn't be.

Sure they can go poke at the source and figure out the problem. But wouldn't it be nicer if the language stopped it happening in the first place?

(and of course this need not stop people being able to poke into the innards if they want to - just provide a separate syntax.)