Greetings,
  The code 'looks' fine, however mIRC tells me its missing a bracket. The re-editing is going slowly trying to remove all the comments and there's some pipeing symbols which I'm not quite sure why you added them, they'll probably have to be removed also where they are next to a comment.
ie:
| ; invalid destination folder
What is the correct command line entry for your alias if it becomes functioning for a mIRC folder named Test and the original Filename Fatext.txt in that folder to split up?
is it 
/Splitup Test Fatext
??
MDA