You say:

    > Undefined subroutine Foo::BarMod::whiz at line 17.
    > ...
    > Well, it's a class method, so the first one doesn't really
    > apply.
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No it's not. That's not a class method--that's a fully-qualified subroutine 'whiz' in the class 'Foo::BarMod'. Very much not a class method. You're saying, "call the function 'whiz' in the package 'Foo::BarMod'". But such a function doesn't exist, so Perl doesn't like that.

If you want inheritance, call it as a class method, which is calling it like this:

    Foo::BarMod->whiz
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(Or indirect object syntax, but that's generally not used much.)

In other words: you came to the right conclusion, but you may have spared yourself quite a bit of confusion and frustration if you had your terms right. Inheritance--ie., checking in @ISA--only occurs for class methods (and object methods). Not for fully-qualified subroutines.