Setting a global variable from another module is also another dangerous practice. Sometimes it is useful, witness Test::More. However it is a fragility in the code. For instance if there is any possibility that your module can be used twice in the same code, with different defaults, you have no sane behaviour.

Therefore I'd advise you to think very carefully before creating APIs that set some global value, let alone trying to make it easy in your API to do that. There are times when it is clearly so unlikely to cause problems that nobody could reasonably object. But those occasions are few and far between. Test::More should be viewed as far more of the exception than the rule in this respect.