If they're class methods (i.e., not instance methods), you can call them as such. For instance: Foo::Bar::Subclass1->s1();will call the s1 you've defined in Foo::Bar unless you override it with a similarly named method in Foo::Bar::Subclass1. In either case, the name of the class that it is invoked from will be passed in as the first argument (instead of a reference to the instance).

If you've already read the docs and they're not satisfactory, I'd suggest reading Damian Conway's excellent book Object-Oriented Perl. It's not introductory, but it's very thorough.