They can actually dehydrate plants' roots. Routine pruning stimulates growth, too. That's not what you want when your main goal for your plants is survival. You may need to do some selective pruning, through when a plant wilts and begins to show leaf scorch, and branches begin to die. In this case, cutting back the top will reduce the water demand the foliage places on the roots.
Some perennials are extremely drought-tolerant.
They can survive long periods without water. If outdoor restrictions keep you from watering anything, cut back perennial flowers that wilt. This will reduce the plant top's demand for water and help keep the root system alive. Lightly pruning shrubs that become severely wilted will also help them conserve water and survive the dry period. The best thing you can do to help plants survive drought is to put mulch over their roots.