As inman suggested you can make use of eval. If you dont have any peroformance issue, you can use s operator.

@letters = qw(A B C D E F);
$string = "AFTYUBEWTWECRTUTYIYTDDDDRYJURTHJTREEEEEFGSDFF";
for  $a (@letters)
{
    $count = $string =~ s/$a//g;
    print "$count\n";
}

output:
1
1
1
5
7
4
[download]