I've had a different interpretation than serf on this. I believe that there is little of the "precious RAM" that is used, (unless that matches were large), since there are only three $&, $' and $` variables. I've always thought that it was more the slightly extra cpu time (hence the "can slow your program down"). I've actually thought that the authors were just trying to be truthful, and tell you what was going on, and rather to pay attention to the sentence that was cut off So avoid $&, $’, and $‘ if you can, but if you can’t (and some algorithms really appreciate them), once you’ve used them once, use them at will, because you’ve already paid the price.  As of 5.005, $& is not so costly as the other two.