If you have another look at perlre you'll see that ?!(mild)) is a look ahead assertion and you need look behind ?<!(mild).

But, at least on my version of perl 5.16 the docs say

 "(?<!pattern)"
               A zero-width negative look-behind assertion.  For examp
+le "/(?<!bar)foo/" matches any 
               occurrence of "foo" that does not follow "bar".  Works 
+only for fixed-width look-behind.
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So you probably need to try a different approach, Try doing your test in 2 phases 1. match lines with active and capture the previous word then 2. skip any line where that word is 'mild' or 'moderate'.

If you show a few example lines of your data then someone here may have a better suggestion.