package A;

  sub new {
      my $pkg = shift;
      bless {}, $pkg;
  }

  sub do_something {
      my $self = shift;

      my $n = $self->x() + $self->y();
      print "something is $n\n";
  }

  sub x { die "subclass must override x\n" }
  sub y { die "subclass must override y\n" }

  package B;

  @ISA = (A);

  sub x { 42 }
  sub y { 47 }

  package main;

  my $object = new B();
  $object->do_something;
  # A::do_something invokes B::x and B::y

##</code><code>##

  package A;

  sub new {
      my $pkg = shift;
      my $self = bless {}, $pkg;
      $self->init();
  }

  sub init { die "subclass must override init()\n" }

  sub do_something {
      my $self = shift;
      my $n = $self->{x} + $self->{y};
      print "something is $n\n";
  }

  package B;

  @ISA = (A);

  sub init {
    my $self = shift;
    $self->{x} = 42;
    $self->{y} = 47;
  }

  package main;

  my $object = new B();
  $object->do_something();
  # A::do_something reaches into the bag, and pulls out
  # numbers that B::init provided