package Automobile;
sub covers_slots {die "Ought to be overridden!"}
__END__
package Car;
our @ISA = qw[Automobile];
sub covers_slots {1;}
__END__
package Truck;
our @ISA = qw[Automobile];
sub covers_slots {3;}
__END__
package ParkingLot;
$slots = $automobile->covers_slots;