Knew I'd forgotten something :-)

Here you go...

use DLMedia::Renderer;
use Test::MockObject::Extends;

my $obj = DLMedia::Renderer->new() ;
$obj = Test::MockObject::Extends->new($obj) ;
[download]

Gives...

Modification of a read-only value attempted at /home/stevewells/perl/lib/perl5/site_perl/5.8.5/Test/MockObject/Extends.pm line 24.
Cheers, Steve