input.txt

ABCDABCDABCD
AAABBBCCCDDD

teste.pl

use strict;
use warnings;
use Data::Dumper;
use List::Util qw(reduce);
use File::Slurp qw(read_file);
 
my $score = reduce { our($a, $b);  $a->{$b} += 1; $a } {}, grep /[ABCD]/,split('',read_file("./input.txt")); 
print Dumper($score);

__END__

$VAR1 = {
          'A' => 6,
          'D' => 6,
          'C' => 6,
          'B' => 6
        };

teste.sh

bash$ grep -o . input.txt | sort | uniq -c 
      6 A
      6 B
      6 C
      6 D