in reply to Returning _which_ pattern matched...?

You could also consider a more difficult example where you are trying to match lines a data that fall into different "types", so you have different regexs to catch them, even though you are trying to collect the same data regardless of the form (clear as mud so far, right?).

Let's just say that you end up with three different regexs. You deside to write three different ones because it would have made the single combined regex a god-awful mess. Good. Now, if you use Hofmator's suggestion, you'll get something like this:

if ($string =~ /re1/ || $string =~ /re2/ || $string =~ /re3/)
where $1 will be populated with what you are looking for, but you still may not know which of re1, 2 or 3 matched. In this case I would use a bigger if. If you want to know which regex matched, it is presumably because you want to do different things with the results. So put them all in if..else's:
if ( $string =~ /re1/ ) { #do thing 1 } elsif ( $string =~ /re2/ ) { #do thing 2 } elsif ( $string =~ /re3/ ) { #do thing 3 }
It is bulkier than other suggestions, but I suspect it is a more generally useful answer to your question.