my @interesting_customers = get_interesting_customer_list();
my @time_periods = (qw( money3am money6am ... ));

# Build a total for customer list
foreach $customer (@interesting_customers) {
    # only looking at the interesting ones
    $myHash{$customer}{money_spent}=0;
    foreach $time_period (@time_periods) {
        $myHash{$customer}{money_spent} += $myHash{$customer}{$time_period}{money};
    }
}

# How much did 'JoeBiden' spend with us?
print "JoeBiden spent $myHash{JoeBiden}{money_spent} dollars with us\n";