Yes that part (the fact that the OP wants to optionally use a module if present) is quite clear. What it is not is why he wants to mock the absence of that particular module in its current setup/environment!

The only thing which I thought would make sense is for testing that optionally loading code logic: I assumed that the problem was testing the module absence, which it can be done having a subroutine which mocks the loading of a fantasy module, e.g.: sub{ use Supercalifragilistichespiralidoso; 1 }