Aristotle already told you what you have to do and the reason why is because automagic assignment to $_ is a short-cut. It's meant as a convience for those situations where you don't really want or need to declare another variable for a simple loop. But when you have multiple nested loops the convience of $_ is largely lost so you should name your loop variables explicitly (all but one of them anyway :).

Things like $_2 are more obfuscatory than useful IMHO