perl-Test-MockObject
Perl extension for emulating troublesome interfaces
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.
Artistic-1.0 OR GPL-1.0-or-later