Hi Karen, salad/spring onion (same thing) and chives are leaf veg, same as leeks, since we want their leafy goodness and not too much basal swelling on the salad onions,
The salad onions we normally sow are not perennial and as you say, one harvest is the end of them. I have not tried cutting their green top to make them resprout.
You can buy certain types of onion which keep dividing at the base. Likewise Babington leek.
And yes, chives are true perennial!
