I think the real key lies in carefully reading what the manpage is telling you. It *wraps* your program with the supplied chunk of code. Without -e, or a file it drops down to STDIN (which of course is not going to be valid code in this case).

A shebang with just -p would then be cat because the remainder of the file is valid perl (type perl on the command line, you're interactive mode. type C-d or F6 etc. (platform dependent) et voila. fini. no complaints)

--
perl -pew "s/\b;([mnst])/'$1/g"