You can assign a list to a hash slice, but that's about it. You need another loop:

my %add = ( a => 1,
            b => 2,
            c => 3,
          );
for (1 .. 10) {
    $totals{$_} += $add{$_} for keys %add;
}
print Dumper(\%totals);
[download]

Or, you use each:

    while (my ($k, $v) = each %add) {
        $totals{$k} += $v;
    }
[download]