For the dipping sauce: Mix the lime juice, sugar and 100 milliliters (1/2 cup) water in a small bowl, stirring to dissolve the sugar. Taste and adjust the flavors if necessary to balance out the sweet and sour. Add the fish sauce, garlic and chiles. Taste again and adjust the flavors to your liking, balancing out the sour, sweet, salty and spicy. Cover with plastic wrap and set aside at room temperature until needed, up to 24 hours. Just before serving, mix in the cilantro and Vietnamese coriander.
For the summer rolls: Place the rice paper wrappers in a heatproof bowl and cover with hot water. Leave to soak until soft and pliable, 5 minutes or per package instructions.
Drain the wrappers on a clean tea towel, then spread out on a work surface. Lay a sawtooth coriander leaf down the middle of each wrapper, trimming the herb to fit if necessary. Arrange 3 prawns per wrapper on top and scatter over some carrots, cucumbers and spring onions. Finish with a couple of leaves of Thai basil, cilantro, Vietnamese coriander and perilla. (Make sure you don't over-fill the papers, as they may split if you do.)
Fold in the sides of the wrappers and then roll up into a neat cylindrical shape. Serve immediately with the herby Vietnamese dipping sauce.
Most large supermarkets now stock Asian ingredients such as rice paper wrappers. Keep wrappers as a store-cupboard ingredient; they are great for a last minute starter or snack -- all you do is soak them in hot water and fill with your favorite ingredients. Just be careful when handling, as they tear quite easily, particularly if overfilled.