You could just make a hash of the letters with the keys the letters and the values the numbers.
Then it would be simply a case of splitting up the string into characters and going through them and if it's a letter it looks it up in the hash.

This is just an example, but it works...:

$string = "AFP254";
$number = 1;
foreach('A'..'Z'){
        $alpha_nums{$_} = $number;
        $number++;
}
                                                                      
+      
@string = split("", $string);

$total = 0;
foreach(@string){
        if(/\S/ && !/\d/){
                $_ = $alpha_nums{$_};
        }
        $total = $_+$total;
}
print "$total\n";
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