sub name {
   my $self = shift;
   if (not defined $self->{name}) {
      $self->{name} = $self->_build_name();
   }
   return $self->{name};
}

##</code><code>##

sub name   { $_[0]->{name}   //= $_[0]->_build_name }
sub colour { $_[0]->{colour} //= $_[0]->_build_colour }
sub owner  { $_[0]->{owner} }           # this one has no default
sub height { $_[0]->{height} //= 2.5 }  # another way to provide a default
...;

##</code><code>##

my $aj      = Horse->new(name => "Applejack");
my $metres  = $aj->height( in => "metres" );
my $inches  = $aj->height( in => "inches" );
my $hands   = $aj->height( in => "hands" );
my $silly   = $aj->height( in => "lightyears" );