What i have eaten just now I would say: WHAT I HAVE JUST EATEN (present perfect with just) it makes reference to an action that happened in a recent past, the action took place recently, a short time ago. I have never used JUST NOW as in your sentence but I suppose that's correct as well. I think it's also correct with a past time action like It happened just know and for sure with the present continuous He's opening the door just now.
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