It's scary because, knowing only one language means that he can only think about, and express solutions to, any problems in terms of that language. In this case

It's scary because, knowing only one language means that he can only think about, and express solutions to, any problems in terms of that language. In this case, he was not expert at Perl either, so the net result was ... not good. I remember seeing Perl programs that would have been better written as very short shell scripts. Note that "mainly just knows Perl" is quite different from "only knows Perl". Since then, I have never met another programmer who knows only one language; I think they are pretty rare. As for proving the case, I don't think I can: there may well exist a good monolingual programmer out there. It's just not likely.