A perfective verb means:
- A one-time action
- a pre-discussed question/deal/action etc
- some specific action for you or someone, and this action is connected with something previous (maybe in the same sentence)

- Ты ел когда-нибудь солянку?
- Я никогда не ел солянку, я поем ее завтра.

- Ты ел когда-нибудь солянку?
- Я никогда не ел солянку, я буду есть ее завтра.

Why do these variants mean the same thing?

The answer lies in meaning of the verb "поем". "Поесть" means an one-time action, but the result can be incomplete. "Я поем солянку" means you will eat part of the солянка or all of that, but no one is sure, and it doesn't matter.
But an imperfective construction "буду есть" means exactly the same, because an imperfective form implies incomplete action.

So, if you use the perfect verb "съесть", it implies that you will eat totally all that you mean. For example:

- Я приготовила солянку, ты ел её?
- Я не ел солянку, но я съем ее завтра. ("съем ее" implies that you will eat all the солянка you have)
- Не ешь всё, оставь мне немного!
- Хорошо, я съем только одну тарелку. (here you specify exactly how much of the soup you will eat)