Internal Program Version: perl-test-mockobject  1.20200122.5.42.0

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.

https://metacpan.org/release/Test-MockObject