gistfile1.pl

use Moose::Util::TypeConstraints;
use Moose::Util ();

my $TYPE_REGISTRY = Moose::Util::TypeConstraints->get_type_constraint_registry;

$TYPE_REGISTRY->add_type_constraint(
    Moose::Meta::TypeConstraint::Parameterizable->new(
        name                    => 'DBref',
        package_defined_in      =>  __PACKAGE__,
        parent                  =>  Moose::Util::TypeConstraints::find_type_constraint('Item'),
        constraint              =>  sub {
            # they can be blank, basically
            return 1 unless(defined($_)); 
            # should be a hashref with at least those two keys present 
            return (
                ref($_) eq 'HASH' && 
                (defined($_->{'$id'}) && defined($_->{'$ref'}))
            ) ? 1 : 0;
        },
        constraint_generator    =>  sub {
            my $type_constraint = shift;
            my $coll_name       = ($type_constraint) ? $type_constraint->name : undef;

            if(defined($coll_name)) {
                return sub {
                    return unless($_->{'$ref'} eq $coll_name);
                    return 1;
                };
            } else {
                return sub { 1; };
            }
        },
    )
);
Moose::Util::TypeConstraints::add_parameterizable_type($TYPE_REGISTRY->get_type_constraint('DBref'));