1. Place allspice, cloves, and cinnamon in the center of a coffee filter or an 8-inch square of cheesecloth. Gather edges into a sack, and tie sack with a small piece of kitchen string. Combine spice sack, pear juice, and pear nectar in a 3-quart slow cooker. Cover with lid, and cook on LOW 3 hours.
2. Throw away the spice bag. Serve hot.