Another way to do it:

use strict;
use warnings;

my @letters = qw(A B C D E F);
my $string = "AFTYUBEWTWECRTUTYIYTDDDDRYJURTHJTREEEEEFGSDFF";
my %chCount;

++$chCount{$_} for split '', $string;
print "$_ = $chCount{$_}\n" for @letters;
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Prints:

A = 1
B = 1
C = 1
D = 5
E = 7
F = 4
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