package X;
sub foo;
sub bar;

package Y;
@ISA = 'X';
sub foo;

package main;
X->foo;  # calls X::foo('X');
X->bar;  # calls X::bar('X');
Y->foo;  # calls Y::foo('Y');
Y->bar;  # calls X::bar('Y');

##</code><code>##

// yes, this is C++, so shoot me
class X {
public:
  virtual int foo () { return 1; }
  int bar () { return 2; }
};

class Y : public X {
  virtual int foo () { return 3; }
};

##</code><code>##

void printStuff (X& obj);

##</code><code>##

package Employee;
sub new {
  my ($class,$fname,$lname) = @_;
  die if $class eq 'Employee';
  bless { FNAME => $fname, LNAME => $lname }, $class;
}

package Employee::Boss;
@ISA = qw( Employee );
sub new {
  my ($class,$fname,$lname) = @_;
  my $self = $class->SUPER::new($fname,$lname);
  $self->{PAY} = 1_000_000;
  bless $self, $class;
}

# and so on...

##</code><code>##

use Employee;
# defines Employee, Employee::Boss,
# Employee::Hourly, and Employee::Intern
$joe = Employee::Intern->new('Joe', 'Schmoe');
$jay = Employee::Boss->new('Jay','Schmay');
$not = Employee->new('Not', 'Happening');       # <-- dies