No, you're correct. It's with the installed modules. But if there was an incompatible change between Foo::Bar v 2.71828 (on the production servers) and Foo::Bar v 3.14159 (on my my dreamhack), and there was an incompatible change between those versions, or a bug in one of them, then my code may suddenly start doing something new and strange in the new environment. Not much we can do about it, except be aware of the possibility, and document it when it happens.