Mocking is a common tool, especially for testing. By strategically
replacing a subroutine, one can isolate segments (units) of code to
test individually. When this is done it is important to know that the
mocked sub was actually called and with what arguments it was called.

Mock::MonkeyPatch injects a subroutine in the place of an existing one.
It returns an object by which you can revisit the manner in which the
mocked subroutine was called. Further when the object goes out of scope
(or when the "restore" method is called) the original subroutine is
replaced.