Despite your claimed guarantee, you didn't guarantee that Net2::Server (or some subclass thereof) won't someday implement a method named foo() that does something radically different from bar(). And then your code will break.

The problem is that it is as much an abuse of an API to assume that a particular method will never be implemented as it is to use an internal one which may change. Also related is that it is a mistake to assume that what you'd think that foo() should mean will be what someone else will think. This is true even for such obvious values of foo() as a constant named PI. Is that a number or a Unicode character? This related mistake is a meta-problem with can().

Instead the right way for you to do that is to have some compatibility layer which guarantees that the interface that you want will be supported. That layer could be something as simple as a hash that says that method X is provided by package Y under name Z. Or you can actually implement a class which proxies the methods that you are interested in (incidentally giving you somewhere to put utility methods that one base class has and the other doesn't).

Now you don't have to worry about the API of Net2::Server changing on you, and you don't need to call can().