"perfect" cube kinda depends on what you want to use the cube for. A perfect isometric cube and a perfect 3 point cube are gonna look totally different but they're both "correct". Also if you have the right technical knowledge, you don't need a still life reference to practice from. Boring answer, but the best way to draw Perfect cubes is to draw 1,000 crappy ones

I don’t believe Marshall actually goes into drawing a “perfect” cube with no distortion in his lectures; although, he does give all the information you will ever need in terms of perspective. If you wish to understand “perfect” perspective Dan Beardshaw has two good videos outlining exactly that. https://www.youtube.com/watch?v=RsbzCHLsQuQ https://www.youtube.com/watch?v=LGAURGTV3v0     These videos probably have too much information though, but that’s kind of the point. Don’t worry about being perfect with your cubes; or simply put, stop drawing like an engineer as soon as you can. Your goal is to be able to see cubes (really anything you want to draw) as a 3D model in your head. So draw from life if it helps you see the 3d model while drawing or draw with perspective lines to help you see the 3d model while drawing, and draw from imagination with no help and then see if it’s correct. That should help you see the 3d model while drawing.     Your ultimate goal is to think entirely in 3d and completely forget you’re drawing on a two dimensional surface. That’s why you learn the cube.

I would do several drawings with perspective lines so you can understand the concept and then draw a whole ton from life so you can get a good intuition of how long sides become at different angles and so on.