The text matched by the used group most-recently closed (i.e. the group with the rightmost closing parenthesis) of the last successful search pattern.
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 }))By setting and then using $var in this way relieves you from having to worry about exactly which numbered set of parentheses they are.
This variable was added in Perl v5.8.0.
Mnemonic: the (possibly) Nested parenthesis that most recently closed.