Current Perl documentation can be found at perldoc.perl.org.

Here is our local, out-dated (pre-5.6) version:

You have to keep track of N yourself. For example, let's say you want to change the fifth occurrence of "whoever" or "whomever" into "whosoever" or "whomsoever", case insensitively.

    $count = 0;
    s{((whom?)ever)}{
        ++$count == 5           # is it the 5th?
            ? "${2}soever"      # yes, swap
            : $1                # renege and leave it there
    }igex;

In the more general case, you can use the /g modifier in a while loop, keeping count of matches.

    $WANT = 3;
    $count = 0;
    while (/(\w+)\s+fish\b/gi) {
        if (++$count == $WANT) {
            print "The third fish is a $1 one.\n";
            # Warning: don't `last' out of this loop
        }
    }

That prints out: "The third fish is a red one." You can also use a repetition count and repeated pattern like this:

    /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;