I'm quite comfortable about leaving the key in and letting EUMM complain
The only problem there is that someone who is inexperienced might not understand that the siginificance of the warning is ... ummm ... not significant at all. And if there's a failure in the test suite, then that warning becomes elevated to the level of "red herring" ... which is why, if it were up to me (which it isn't), the 'LICENSE' parameter would not be passed to a version of EU::MM that didn't understand it.
use ExtUtils::MakeMaker 6.31;
ExtUtils::MakeMaker version 6.31 required--this is only version 6.30 a
+t - line 1.
BEGIN failed--compilation aborted at - line 1.