How do I change the Nth occurrence of something?

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

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