
Originally Posted by
gRomoZeka
It's possible to use a perfective verb, but the meaning will be different.
"Хоть гроб заказывай" does not speak of a real intention to order a coffin, it's an indignant/distraught exclamation of sorts. A person speaks about an imaginary action, something s/he isn't ready to do (at least, yet).
Changing the verb to perfective, you make it an advice: "Хоть гроб закажи" = "At least order a coffin"
Look at similar examples with a verb "заводить":
Imperfective -> Все, у кого нет домашних животных, будут облагаться налогом. Хоть собаку заводи! = Everyone who doesn't own a pet will be taxed. ~ It seems the only option left is to get a dog!
Perfective -> Ты так одинок. Хоть собаку заведи! =You are so lonely. At least, get a dog!