Line a baking sheet with plastic wrap. Wearing rubber gloves, wash the cactus pears and with paper towels, rub off any prickly fuzz left on the skin. Halve the pears lengthwise. Carefully scoop out the flesh and seeds and put them into a blender, leaving the thick shells intact. Put the shells onto the baking sheets and freeze them until hard, about 1 hour.
Add the syrup and lime juice to the blender and puree until smooth. Taste and adjust the flavor with more syrup or lime juice, to taste. Strain the puree through a fine sieve, discarding any seeds and pulp.
Pour the sorbet into the frozen shells, smoothing the surface so they are level. Cover with plastic wrap and return the filled shells to the freezer for at least 6 hours. Frozen sorbet shells may be wrapped individually in plastic wrap and kept in the freezer for 1 week.
Put the sugar and water into a saucepan over medium-high heat and bring to a boil, stirring to dissolve the sugar. Remove from heat and cool before using. Syrup can be stored, covered, in the refrigerator, for 1 month.