I've not played with 65mm film, but I have got a little bit of perfed 70mm (like would go in Hasselblad backs, etc.) and cutting it down to 120 size would leave little nibbles of perfs at the edges. They'd be outside of the imaging area, of course, like they are when using one of those 70mm backs, but they might be annoying. (I actually have a 70mm-to-120 slitter, but haven't used it yet.)
So I would guess 65mm movie film would have the perfs actually slightly into the imaging area, which would be even more annoying. But you're never going to find short ends of 65mm film, and you'd be *shocked* at the price for it new... and that's if it's even available new in single roll quantities. A lot of it has to be bought by the case. So that's going to hurt!