Pour the mezcal in a small saucepan and bring to a boil. Reduce by 1/3. Remove from the heat and let cool slightly.
Scoop the prickly pear pieces into a blender or food processor along with the water, sugar, and reduced mezcal. Puree, then press through a fine mesh strainer. Refrigerate for several hours. Stir well to combine the juices, then pour to an ice cream maker. Follow the instructions according to your machine. Transfer the finished sorbet into a airtight container. You can serve it right away or place it in the freezer until you're ready to serve.