use experimental 'signatures';
sub length_at_position_between($s, $p, $n = "\n") {
    my ($prev, $next);
    while ($s =~ /$n/g) {
        $prev = $next;
        $next = pos $s;
        last if $next > $p;
    }
    return $next - $prev;
}

print length_at_position_between
    ("test\nI want length of this line\n test", 12); # 27