As mentioned above, the access restrictions in Perl are more about manners than the loaded shotgun of Java or C++. However, in looking at your post it drew a few parallels (for me at least) with the way classes operate in Java. In otherwords, only one public class per file.

Personally I don't think theres anything wrong in having more than one class per file provided one class (and one only) is considered public, as in Java. The others are then private (manners permitting) and only exist to support the public class. And thats the important point. If My::B were to support anything other than My::A then it would have its own file.

I can think of a few examples of where this is done, HTML::Template being one of them (if memory serves me correctly at least).