Perennial food sources receive an emphasis in permaculture because of their ability to produce food reliably with minimal inputs, in a variety of conditions. Compared to traditional annual vegetables, many perennial food sources are more sustainable, more reliable, and just flat out easier to grow.
With the same amount of inputs, perennial crops produce higher yields over the course of a longer growing season. And they require much less work on the part of the gardener or farmer. Getting more returns for less work is a big part of what permaculture is all about.
Annual plants extract massive amounts of nutrients and water from the soil, and their thin shallow roots require a finely cultivated soil in which to grow. Perennial crops balance this aspect of gardening. The roots of perennial food sources grow deep, breaking up compacted soil to find water deep in the ground and bringing up water and nutrients to share with the other plants around them.
If you're already growing annual veggies in raised beds or in traditional garden rows, that's probably not the best place to start adding in perennial crops. Perennials don't require as much special care as annuals, so save your prime spots for the more difficult plants.
Perennial crops don't require nearly as much water as annual crops, so they probably don't need to be included in your primary irrigation system. Depending on your climate and your design, perennial food sources might not require any irrigation at all. You can help to make that happen by using permaculture earthworks designs like berms and swales and hugelkultur.
Try working some perennials into your plan along the fringes of your garden, outside your primary irrigation and outside the area where soil has been intensively cultivated. If you have an open area on your property where you would like to see lush edible growth and a thriving ecosystem, consider installing a food forest full of perennial food sources.
For canopy plants, the most popular perennial food sources are fruit and nut trees. Fruits like peaches, plums, persimmons, and loquats provide an abundance of food with minimal water and care. Nut trees like pecans and chestnuts add wonderful diversity to the permaculturist's diet, and they typically fetch a good price at market.
Several great perennials are available for the understory layer. You can grow dwarf fruit trees, although these will likely require a bit of maintenance. Native persimmons, wild plums, and shrubs that produce edible berries are popular choices. Think outside the box, and work in plants that are useful for teas, medicines, and dyes like roses, yaupon hollies, and Osage orange trees.
Below the understory, you can grow brambles like raspberries and blackberries. In sunnier locations you can grow perennial vegetables like artichokes, asparagus, and perennial kale. Sunchoke, or Jerusalem artichoke, has become famous in permaculture communities for its prolific growth with no special care.
Finally, in the herbaceous layer along the ground, there are many fine edibles and other useful perennials that you can use as a green mulch to cover the ground. Try sorrel, a wonderful perennial salad green that is one of the earliest plants to yield food each spring. Encourage edible weeds like lamb's quarters and shepherd's purse. Wild onions, wild leeks, chufa (nutsedge), and groundnut. The world is full of edible plants, and learning about them is one of the most rewarding aspects of studying permaculture.
If you'd like to learn more about perennial food sources, and the various ways you can work them in to your own garden design at home, there are many resources available to help you on your way.
For information about the plants - the various perennial food sources that you have to choose from - see Eric Toensmeier's book Perennial Vegetables. For information about the methods - the principles you should follow and the general approach to planning - see Toby Hemenway's book Gaia's Garden or check out his online training Food Forest Design.