###
### delivery-charge
###

## Chapter 3 section 7.5

sub delivery_charge {
  my ($quantity_ordered) = @_;
  my ($hour, $day_of_week) = (localtime)[2,6];
  # perform complex computatation involving $weight, $gross_cost,
  #     $hour, $day_of_week, and $quantity_ordered
  # ...
  return $delivery_charge;
}