Here is a contrived example:

#!/usr/bin/env perl
use 5.010_000;
use strict;
use warnings;

{   package Polygon;
    use Moose;
    my $SIDES = 10;
    sub sides {
        my ($self, $new_sides) = @_;
        # Class or Instance ...
        if ( ! blessed $self ) {
            $self = $self->new; # This is a bit of a hack ... obviousl
+y =)
        }
        # Set sides
        if ($new_sides) {
            $self->{_SIDES} = \$new_sides; # You can make $SIDES reado
+nly here.
        }
        # Get sides
        return ${ $self->{_SIDES} };
    }
    sub BUILD {
        my ( $self, $params ) = @_;
        $self->sides( $params->{_SIDES} // $SIDES ); 
        return $self;
    }
    no Moose;
    1;
}

{   package Triangle;
    use Moose;
    use Carp qw(carp);
    extends qw(Polygon);
    sub BUILD {
        my ( $self, $params ) = @_;
        if ( $params->{_SIDES} ) {
            carp sprintf 'Cannot alter SIDES with value (%s)', $params
+->{_SIDES};
        }
        $self->{_SIDES}       = \3; # ALWAYS 3 SIDES
        return $self;
    }
    no Moose;
    1;
}

say {*STDERR} Polygon->sides;                       # 10
say {*STDERR} Polygon->new->sides;                  # 10
say {*STDERR} Polygon->new( _SIDES => 20 )->sides;  # 20

say {*STDERR} Triangle->sides;                      # 3
say {*STDERR} Triangle->new->sides;                 # 3
say {*STDERR} Triangle->new( _SIDES => 4 )->sides;  # 3 with warning m
+essage
[download]

UPDATE: Just seen stvn's post ... do that instead =)

UPDATE: To do this with stvn's recommendation, this example would look like so:

#!/usr/bin/env perl
use 5.010_000;
use strict;
use warnings;

{   package Polygon;
    use Moose;
    use MooseX::ClassAttribute;
    class_has 'sides' => (
        is      => 'rw',
        isa     => 'Int',
        default => sub { 10 },
    );
    __PACKAGE__->meta->make_immutable();
    #MooseX::ClassAttribute::containing_class()->meta()->make_immutabl
+e();
    no Moose;
    no MooseX::ClassAttribute;
    1;
}

{   package Triangle;
    use Moose;
    use MooseX::ClassAttribute;
    extends 'Polygon';
    class_has '+sides' => (
        default => sub { 3 },
    );
    __PACKAGE__->meta->make_immutable();
    #MooseX::ClassAttribute::containing_class()->meta()->make_immutabl
+e();
    no Moose;
    no MooseX::ClassAttribute;
    1;
}

say {*STDERR} Polygon->sides;       # 10
say {*STDERR} Polygon->new->sides;  # 10

say {*STDERR} Triangle->sides;      # 3
say {*STDERR} Triangle->new->sides; # 3

exit 0;
[download]

Don't know why the commented line #MooseX::ClassAttribute::containing_class()->meta()->make_immutable(); does not quite work. Its an open ticket on the RT queue =)

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