research shows that the "can" method (or allegedly exported sub) does not really exist in any module - documentation says it is built into perl

I suppose it all depends on what you mean by "built into perl." It's true that it's part the perl core, and can be relied on being there. But that's only because UNIVERSAL is the base class for all classes. If you run perldoc -f can, or browse through perlfunc, you will see that can is not a builtin function in the strict sense of the term.