Quote

# $LAST_SUBMATCH_RESULT
# $^N

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. (Mnemonic: the (possibly) Nested parenthesis that most recently closed.)

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

1. (?:(...)(?{ $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 is dynamically scoped to the current BLOCK.

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if ($line =~/S\/R Sending command\r?\n-+\r?\nNRT CC\/(\d+)/) {
    print $1;
}