Start cutting back herbaceous perennials in autumn once they've finished flowering. Some Perennials, such as Achillea, Eryngium, Acanthus and Poppy, have attractive seed heads throughout the winter, so you can choose to leave those in place until the following spring.
Perennials all finish flowering at different times in the autumn, so its a good idea to cut them back as and when they go over, so any flowers can be enjoyed in your garden for a s long as possible. When you do cut Perennials back, the same rule applies to most. Cut the stem (with Secateurs) right down to 1 inch from the ground. You may be able to see new leaves growing at the base of the plant already, that's next summer's flowers!
When tidying up your borders don’t be in too much of a rush to cut everything back before the onset of bad weather. Hardy Fuchsias and perennials such as Penstemons will benefit from having the stems left on over winter to give some protection to the crown of the plant. Cut these stems hard back in early spring once you see the first new buds breaking from the base of the plants. Plants with decorative seed pods such as Love in a Mist look really attractive during winter and help feed the birds too and many grasses look spectacular when it’s frosty, especially the taller varieties.