try this one:

$input =~ s{\Q.\E}{\Q\.\E}g;
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gives

Kev\.has\.a\.stupid\.perl\.question
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this \Q and \E tags advise the RE engine to ignore magic characters and threat everything between them as a string (I hope a better explanation is somewhere in the tutorials see: http://www.perlmonks.org/?node=Tutorials#Pattern-Matching-Regular-Expressions-and-Parsing)