To enter a Perceptoscope, you approach it and press Z. To exit, you use the ESC menu and select "Return to Gallery" - though, as reported, if you can't access ESC Menu, you won't be able to leave the Perceptoscope. As mentioned, we'll explore alternate inputs to exit Perceptoscopes outside of a menu, though, ideally, we'd have you able to access the menu in the first place!