My rhubarb is forming a flower head. Should I cut it off?

Probably because the weather has changed and it thinks it's time to bloom.

Perennial means that the plant will come back every year - not that it produces or remains green all the time. It will die back and regrow the next year. Annuals die after producing and do not come back (regrow).

https://www.gardeningknowhow.com/edible/vegetables/rhubarb/rhubarb-bolting.htm

Hi LaVonne,

Since rhubarb is grown for the stems, most gardeners choose to remove the flowers as soon as they appear so the plant can focus its energy on leaf growth. Rhubarb flowers can simply be cut from the plant as soon as you see it appear. If your rhubarb produces a flower, this does not affect the stems and leaves.

Maybe trhis video will help...

Yes, you need to remove the flower heads as they take nourishment from the stalks.