Yes, they are perennial and multiply at the base to form clumps, which can be divided or left depending on how/where you'd like them to grow. The more that populate the clump, the smaller each individual onion will be. They do not bulb up and are eaten like spring onions, leaves and all. You can earth up to make the white shank longer.
When they go dormant, which I've witnessed in both summer and midwinter, they die back and form little bulbs underground, resuming growth when conditions are favourable.
