#!/usr/bin/perl
use 5.014;

# 986041

my $str="BATCATDATEFEAT";

my $A=0;
my $T=0;

my $output = 'D:/_Perl_/PMonks/986041.txt';

open(RESULT,">>$output") or die "Can't open $output, $!";

while ($str=~ /A/ig) {
    $A++;                           # OP's Line 4
    print "\n A=$A ends at ",pos $str,"\n";
    print RESULT "\n A=$A ends at ",pos $str,"\n";
}

while ($str=~ /T/ig) {
    $T++;
    print "\n T=$T ends at ",pos $str,"\n";
    print RESULT "\n T=$T ends at ",pos $str,"\n";
}
close(RESULT);                    # originally, Line 17  
exit;

=head execution:

C:\>type d:\_Perl_\PMonks\986041.txt

 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

=cut