$_ = "ATGCCCGATATATATCCCNNNATATATGCGCATGCTGCT";

my ($l, $m) = (-2, 0);
$m = length $', say -$l - 2 + ($l = length($`)) while /AT/g;
say $m;
__END__
0
5
0
0
0
6
0
0
4
6