I'm late on this but here's a version I liked, for brevity:

use strict;
use warnings;

my $str = "aabceezazerrfde";
my %wanted = (a => 7, e => 2, z => 26);
my $tot = 0;
$wanted{$_} && ($tot +=  $wanted{$_}) for (split //, $str);
print "Total $tot\n";
[download]

Better in some circumstances might be:

exists $wanted{$_} && ($tot +=  $wanted{$_}) for (split //, $str);

but I don't think it matters in this case. I discussed mixing "for" and "if" in one line with moritz and shmem recently, and I rather like it, though someone said it was too much logic for one line...

 ($tot +=  $wanted{$_})  if $wanted{$_} for (split //, $str);

never seems to work...

Cheers