for, map, grep and split provide a little synergy here:

use strict;
use warnings;

my $str = "aabceezazerrfde";
my %wanted = (a => 7, e => 2, z => 26);
my $tot = 0;
my $counter = 0;

$tot += $_ for map {++$counter; $wanted{$_}} grep {exists $wanted{$_}}
+ split //, $str;
print "Found $counter letters totaling $tot\n";
[download]

Prints:

Found 9 letters totaling 81
[download]

Update: s/,/;/ in map - bug fix, thanks toolic.