Your analysis is incorrect. It has nothing to do with the module having already been loaded.

That tries to load a file named MAS::Global (not MAS/Global.pm), and no such file has been loaded. Furthermore, it wouldn't be an error if it had been.

The error is because you used require 'MAS::Global'; instead of require MAS::Global;, just like we said. (require 'MAS/Global.pm'; would also work.)