I don't know anything about xevian's situation, but one place where I can see something kind of like this come up is when there are backward compatibility problems. For example, let's say DBD::XYZ version 1 only supports XYZ database version 1, whereas DBD::XYZ version 2 only supports XYZ database version 2. If your code, for whatever reason, might have to connect to instances of different versions of XYZ within a single program, you're going to have a problem.