## backstops (should be overridden)
sub default_color { "brown" }
sub sound { croak "subclass must define a sound" }

Next comes the methods that work with either a class or an instance:

## class/instance methods
sub speak {
  my $either = shift;
  print $either->name, " goes ", $either->sound, "\n";
}
sub name {
  my $either = shift;
  ref $either
    ? $either->{Name}
    : "an unnamed $either";
}
sub color {
  my $either = shift;
  ref $either
    ? $either->{Color}
    : $either->default_color;
}

Finally, the methods that work only for the particular instance:

## instance-only methods
sub set_name {
  ref(my $self = shift) or croak "instance variable needed";
  $self->{Name} = shift;
}
sub set_color {
  ref(my $self = shift) or croak "instance variable needed";
  $self->{Color} = shift;
}
}

Now that you have your abstract base class, define some concrete classes that can have instances:

{ package Horse;
  our @ISA = qw(Animal);
  sub sound { "neigh" }
}
{ package Sheep;
  our @ISA = qw(Animal);
  sub color { "white" }    # override the default color
  sub sound { "baaaah" }   # no Silence of the Lambs
}

Finally, a few lines of code to test your classes:

my $tv_horse = Horse->named("Mr. Ed");
$tv_horse->set_name("Mister Ed");
$tv_horse->set_color("grey");
print $tv_horse->name, " is ", $tv_horse->color, "\n";
print Sheep->name, " colored ", Sheep->color, " goes ", Sheep->sound, "\n";