Note also that use of the $& regex special variable or its "friends" $` $' anywhere in a script imposes significant regex matching performance penalties everywhere in the script.

Yeah, this used to be true, but has been slightly improved overtime, and has been basically fixed with Perl 5.20. So the penalty may or may not be there depending of the Perl version being used.

Having said that, I've personally never used those variables for anything else than just testing a couple of times what they do (if only because, at least at work, I'm stuck with a much older version of Perl).