Scroll through the comments. Someone quoted a MASH scene that uses it. It's a TV show and not a movie, but at least it's an example in the wild.

MASH is culturally important but it’s not The Guns of Navarone or A Bridge Too Far. If Bridge on the River Kwai had watches, Sir Alec Guinness would have not yelled hack.

I've seen practically every B-grade and up WWII movie made in English, and I don't remember ever seeing the use of "hack". Anybody please point to movie references if they know any.

My memory isn’t what it used to be but I think this is the first time I’ve heard it too, if that wasn’t already clear from my tone above.

I've never heard "hack" in this context either. "On my mark!" is what I think of.