TIMTOWTDI, the easiest is to print a control-M (\r) before you print your update. HOwever, you shoul dbe aware that perl buufers it's ouput and it'll bite you with this idf you're not careful. You should disable buffering by doing $|++.

UPDATE</b: And of course dropping the newline as others (inc. Speedy) have pointed out.

--
perl -pe "s/\b;([st])/'\1/mg"