But then, why stop at testing $^W? How about $*? $"? @ARGV?, %SIG? $package::var?
I think you meant this in jest, but this would be a very valuable resource if it could be automated; kind of like the sort of utility that tests for memory leaks in C.
Just being able to know that a given module has tampered with your { global variables/function definitions/namespaces/whatever} is very, very handy.
Perl gives you a lot of rope to hang yourself (and others!) with; having tools to keep it coiled neatly would really be nice sometimes. :-)
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Ytrew