Starting with your data, I get a solution similar to toolic's, but I use List::Util and List::MoreUtils.

use List::Util qw( sum );
use List::MoreUtils qw( zip );

my $str = "aabceezazerrfde";
my $wanted = "aez";
my @values = (7,2,26);

my @wanted = split //, $wanted;
my %value_of = zip @wanted, @values;
my $tot = sum map { $value_of{$_} || 0 } split //, $str;
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Note that if you use warnings (as you should), this will puke up "Use of uninitialized value in subroutine entry" for every letter in $str that is not in $wanted, but otherwise it works fine.

Update: With a small change, no more warnings.