use Modern::Perl;
use Sort::Naturally;

my ( $str, %i, %positions ) = 'BATCATDATEFEAT';

$str =~ s/(A|T)/$positions{$1 . '=' . ++$i{$1}}=pos($str)+1/ige;

open my $fh, '>', 'Results.txt' or die $!;
say $fh "$_ ends at $positions{$_}" for nsort keys %positions;
close $fh;