It's a simple program that doesn't use any other modules, and those are easy to
test. More often, testing a program completely means faking up input to another
module, trying to coax the right output from something you're not supposed to
be testing anyway.

Testing is a lot easier when you can control the entire environment. With
Test::MockObject, you can get a lot closer.

Test::MockObject allows you to create objects that conform to particular
interfaces with very little code. You don't have to reimplement the behavior,
just the input and the output.