That does indeed sound very simple. I think I will try my photodiode module on an analog input of my Arduino one of these days (not quite sure it supports that, though). Then I could possibly set a different threshold which might just get my test setup to work.
Did you have any issue with your laser beam being “larger” than the sprocket holes? Are you using a pinhole or something similar? Maybe this is also fixed simply by using a custom threshold.
Also, are you detecting your laser beam through the film or by reflecting it off of the film?