has 'foo' => ( 
    is       => 'bare', # no accessors
    init_arg => undef, # no longer accessible via constructor
);

##</code><code>##

# One reasonable constructor interface for a square:
Square->new( center => [ $x, $y ], width => $w );

##</code><code>##

package Square;
use Moose;

has 'top_left' => ( init_arg => undef, accessor => '_top_left' );
has 'top_right' => ( init_arg => undef, accessor => '_top_right' );
has 'bottom_left' => ( init_arg => undef, accessor => '_bottom_left' );
has 'bottom_right' => ( init_arg => undef, accessor => '_bottom_right' );

sub BUILD {
    my ($self, $params) = @_;
    my $center = $params->{center};
    my $width  = $params->{width};
    $self->_construct_coordinates_from_center_and_width( $center, $width );
}