I don't know where you are going with this but the $^N variable, but it might be what you are looking for (granted I am aware that it does not get you the number). According to perlvar:

This is primarily used inside (?{...}) blocks for examining text recently matched. For example, to effectively capture text to a variable (in addition to $1, $2, etc.), replace (...) with

(?:(...)(?{ $var = $^N }))
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By setting and then using $var in this way relieves you from having to worry about exactly which numbered set of parentheses they are.

-enlil