You can replace the whole if-elsif paragraph with just

my %count_of;
if (/([ACDEFGHIKLMNPQRSTVWY])/) {
    $count_of{$1}++
}
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As for the result, do not use double quotes around arithmetical operations. With the hash suggested above you can just:

print RESULT  join ' ',
    $id,
    map $count_of{$_}/$tt, qw/A C D E F G H I K L M N P Q R S T V W Y/
+;
print "\n";
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