I say, "when in doubt, map stuff." I used a hash to map letters to values... I also go 81 as an answer. You could also create your two arrays (wanted and values) and then make them into a hash before running this code:

use strict; 
my $str = "aabceezazerrfde";
my %values = ('a' => 7, 'e' => 2, 'z' => 26);
my $total = 0;

foreach(split '',$str){
    if($values{$_}){
        $total += $values{$_};
    }
}
print "Total: $total";
[download]