While it's generally handy if you show some example input, in this case your question is "why is $2 set?" which doesn't need sample input. The answer is because it's the second parenthesised value in the regexp. Perl's REs never compress the list of found values in case knowing which one is which is important. In this case, a simplification will get you what you want:
This way, there is only one set of capturing parens. The first set of parens has the ?: modifier which says "this is for grouping only, not for capturing."