If script A does something useful, then you can't do this.

My first inclination would be to move the variable from script A to module C, use it in script A, and use it in script B. Alternately you can set logic in script A saying, "Don't do anything if variable $x is set", and set it in script B before doing script A.

I'm pretty sure that if I saw the actual scripts in question I could come up with a much better solution than the two suggestions I gave you off of the top of my head.