tye has already told you why you could see slowness caused by backtracking. Also, I would write "seperate $string in two parts: the first 200 words ($intro) and the rest ($rest)" like this:

(Assuming your string starts with a word character and you don't mind the extra non-word character(s) after the 200th word.)

— Arien

Edit: If you don't care about changing the value of $string you could obviously leave out the copy to $rest.