You might as well say: "What we perceive the earth to be made of rock, but it's actually made of whipped cream outside of that perception?"

The shape of the earth has innumerable physical consequences, all of which we observe and measure to be consistent with it being approximately spherical. If you want to say that it behaves in every observable way like a sphere, but somehow is "really" flat, then the definition of "reality" you're using is not a very useful one.

Say more if you don’t mind; it seems like the opposite to me.

I perceive (sense) a flat earth, but outside of my perception it is demonstrated to be a globe.

What if your family has been replaced by very realistic-seeming impostors?

That's why you don't just rely on your senses. You rely on stuff like measuring and mapping and putting satellites in orbit around it.

And so we know. No, it's not "flat outside that perception". It's a globe, full stop.