use Modern::Perl;

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

$str =~ s/(A|T)/$positions{pos($str)+1}=$1/ige;

open my $fh, '>', 'Results.txt' or die $!;
say $fh "Position of $positions{$_} ends at $_"
  for sort { $a <=> $b } keys %positions;
close $fh;