Current Perl documentation can be found at perldoc.perl.org.
Here is our local, out-dated (pre-5.6) version:
Because once Perl sees that you need one of these variables anywhere in the program, it has to provide them on each and every pattern match. The same mechanism that handles these provides for the use of $1, $2, etc., so you pay the same price for each regexp that contains capturing parentheses. But if you never use $&, etc., in your script, then regexps without capturing parentheses won't be penalized. 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.