Try putting use Constants; in the beginning of the A module. Update: I mean in the beginning of both your A and B modules.

(I hope you don't really use B as a module name, as it's used up.)

Update: Your problem might also be that you don't have a package declaration in the Constants module, so the constants get defined in the wrong package (A or B). Also you'll have to Export the constants from the Constants module if you want to use them without a package prefix.