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;

##</code><code>##

Position of A ends at 2
Position of T ends at 3
Position of A ends at 5
Position of T ends at 6
Position of A ends at 8
Position of T ends at 9
Position of A ends at 13
Position of T ends at 14

##</code><code>##

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;

##</code><code>##

A=1 ends at 2
A=2 ends at 5
A=3 ends at 8
A=4 ends at 13
T=1 ends at 3
T=2 ends at 6
T=3 ends at 9
T=4 ends at 14