package Foo;
use Moose;

has 'bar' => (
    is       => 'ro',   # access is read-only
    init_arg => undef,  # you cannot set it via the constructor
    lazy     => 1,      # don't allocate a slot for it unless you access it
    default  => 42,     # and provide the default value
);

##</code><code>##

package Bar;
use Moose;

extends 'Foo';

has '+bar' => (default => 100);