Thor, please elaborate on why "chances are" there's no need for access control in a system that big. Perhaps you interpreted the word "enforcing" in an absolute sense? This is open source: all access control is advisory.

Exposing a member variable in a public API has the disadvantage of freezing an implementation detail. Standard OO theory says that's probably not a good idea in either large or small systems. The distinction between large and small is in the internal API -- it's more important that components in a large system do not violate encapsulation of other components.

Say you have only two small hash-based classes in your distro and one accesses a hash member in the other directly. When you refactor and change the name of that variable, you can just check the other file and be sure that there aren't any ill consequences. In a large system, that's not feasible because it's prohibitively time-consuming and difficult to inspect all the code in the project every time you change an implementation detail. However, with the ACL policy I have in place, I can change how any hash member or leading-underscore sub behaves and feel confident that I only have to check the file I'm currently editing.