## Additional Instance Variables in Subclasses
{ package Animal;
       sub named {
               my $class = shift;
               my $name = shift;
               bless \$name, $class;
       }
       sub name {
               my $either = shift;
               ref $either
                       ? $$either : "an unamed $either"; # see above section for usage
       } 
       sub speak {
               my $either = shift;
               print $either->name," goes ",$either->sound,"!\n";
       }
       sub eat {
               my $either = shift;
               my $food = shift;
               print $either->name," eats $food.\n";
       }

        sub DESTROY {
                my $self = shift;
               print "[",$self->name," has died.]\n";
        }
}
{ package Horse;
       our @ISA = qw/Animal/;
       sub sound { "neigh"; }
        sub DESTROY {
                my $self = shift;
                $self->SUPER::DESTROY;
                print "[",$self->name," has gone off to the glue factory.]\n";
        }
}
{ package Sheep;
       our @ISA = qw/Animal/;
       sub sound { "baaah"; }
}

{ package Cow;
      our @ISA = qw/Animal/;
}

sub feed_a_cow_named {
        my $name = shift;
        my $cow = Cow->named($name);
        $cow->eat("grass");
        print "Returning from the subroutine.\n";       # $cow is destroyed here
}
{ package Barn;
        sub new { bless [], shift }
        sub add { push @{+shift}, shift }
        sub contents { @{+shift} }
        sub DESTROY {
                my $self = shift;
                print "$self is being destroyed...\n";
                for($self->contents) {
                        print " ",$_->name," goes homeless.\n";
                }
        }
}
{ package Barn2;
        sub new { bless [], shift }
        sub add { push @{+shift}, shift }
        sub contents { @{+shift} }
        sub DESTROY {
                my $self = shift;
                print "$self is being destroyed...\n";
                while(@$self) {
                        my $homeless = shift(@$self);
                        print "  ",$homeless->name," goes homeless.\n";
                }
        }
}

{ package RaceHorse;
        our @ISA = qw/Horse/;
        ## extend PARENT constructor
        sub named {
                my $self = shift->SUPER::named(@_);
                print "DEBUG: $self\n\n";
                $self->{$_} = 0 for qw/wins places shows losses/;
                $self;
        }
        sub won { shift->{wins}++; }
        sub placed { shift->{places}++; }
        sub showed { shift->{shows}++; }
        sub lost { shift->{losses}++; }
        sub standings {
                my $self = shift;
                join ", ", map "$self->{$_} $_", qw/wins places shows losses/;
        }
}

my $racer = RaceHorse->named("Billy Boy");
# record the outcomes: 3 wins, 1 show, 1 loss
$racer->won;
$racer->won;
$racer->won;
$racer->showed;
$racer->lost;
print $racer->name," has standings of: ",$racer->standings,".\n";

##</code><code>##

Not a HASH reference at notes_chap_10.perl line 303.
##</code><code>##

$self->{$_} = 0 for qw/wins places shows losses/;