package My::Base;
use Moose;
has filename => (is       => "ro",
                 isa      => "Str",
                 required => 1);
has config => (is => "ro");
no Moose;

package My::Derived;
use Moose;
extends "My::Base";
has "+config" => (lazy    => 1,
                  default => sub {my $self = shift; $self->filename . ".cfg"});
no Moose;

package main;
my $x=My::Derived->new(filename=>"/etc/myconfig");
print $x->config,"\n";'

##</code><code>##

/etc/myconfig.cfg