The transliteration does not interpolate the variable as you expect in your sample. The following illustrates the use of eval. 
use strict;
use warnings;

my @letters = qw (A B C D E F);
my $string = "AFTYUBEWTWECRTUTYIYTDDDDRYJURTHJTREEEEEFGSDFF";
my $count;
foreach my $a(@letters) {
    eval "\$count = \$string =~ tr/$a//";
    print "$count\n";
}

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You can also use a substitution in order to interpolate but avoid the eval. 
$count = $string =~ s/$a//g;

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Both methods change the string. A more flexible way to count the values would be to split the string and use a hash to store the values. 
my %count;
$count{$_}++ foreach split //, $string;
print "$count{$_}\n" foreach (@letters);

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In reply to Re: This should be easy: counting letters in a string by inman
in thread This should be easy: counting letters in a string by Anonymous Monk

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