Inheritance goes one way: up. If B and C are both As, and C->meth is called but C doesn't define a meth, B will certainly not be called! Why should it; after all, C is not a B.

But you say you have "a bunch of code that @ISA = qw(Foo::Bar) but there is no Foo/Bar.pm; it's just a convenience". Convenience for what? It's wrong semantically to inherit from something that doesn't exist.

Note that not having a file Foo/Bar.pm is perfectl all right: Foo::Bar may be defined in another file, or created on the fly at run time. So long that it is compiled before you call any of its methods, you are syntactically safe. You'd better make sure it finishes compiling before its subclasses are defined too, unless you know what you're doing (== funky dynamic stuff)