package VendingMachine;
use base qw(Persist);

...

sub _loaded {
	my ($self, $item) = @_;
	# returns true if $self has a $item loaded
	# in the machine
};

sub vend {
	my ($self, $item, $money) = @_;
	die "no items in stock" unless $self->_loaded($item);
	die "not enough cash" if $money < $item->cost;
	$self->credit($money)->eject($item);
	if ($self->{_change} and $money > $item->cost) {
		$self->give_change($money - $item->cost);
	};
};