Is an onion perennial

No, the most familiar onions are not perennials, but yes, there are onions that are perennials.

Specifically, many familiar onions are biennials. But they are treated as annuals in that their bulbs are used for one growing season when their life cycle allows them to survive two. There are some onions - such as the Egyptian or potato onion - whose life cycle spans more than two growing seasons. They therefore are considered perennials.