my $BC = CPU::Emulator::Z80::Register16->new(
  get => sub {
    return 256 * $self->register('B')->get() +
                 $self->register('C')->get()
  },
  set => sub {
    my $value = shift;
    $self->register('B')->set($value >>8);
    $self->register('C')->set($value & 0xFF);
  }
);

##</code><code>##

$AF = _derive_register16($self, qw(A F));
$BC = _derive_register16($self, qw(B C));
$DE = _derive_register16($self, qw(D E));

...

sub _derive_register16 {
  my($self, $high, $low) = @_;
  return CPU::Emulator::Z80::Register16->new(
    get => sub {
      return 256 * $self->register($high)->get() +
                   $self->register($low)->get()
    },
    set => sub {
      my $value = shift;
      $self->register($high)->set($value >>8);
      $self->register($low)->set($value & 0xFF);
    },
  );
}