my @letters = qw (A B C D E F);
my $string = "AFTYUBEWTWECRTUTYIYTDDDDRYJURTHJTREEEEEFGSDFF";
foreach (@letters) {
    my $count = 0;
    $count ++ while $string =~ /$_/g;
    print "$_ : $count\n";
}

output:
A : 1
B : 1
C : 1
D : 5
E : 7
F : 4