perp with a gun on me, robbery over, looking like he was going to shoot.
All odds at this point are 50-50.
Either you will get shot or you won't.
You will either survive or you won't.
If you do something it may get you killed.
If you do nothing, then you will be killed.
Hence ... do something!
Once you recognize your assailant is going to seriously hurt you (or kill you), anything you do increases your odds for survival.
Look beyond him, open eyes in surprise and say something like "Hit him NOW!" as you move laterally.