The 2 approaches I see here are more:
- setting a inline section or a POD-like documentation to display as help, or
- properly defining a
.manfile to add to your localmanstructure
I honestly don't see the point of having a separate file for that kind of help, unless you have a very big tool and the interface/GUI is already in different file(s).
So stay "plain and simple": all in one file regarding your command-line frontend.
You can still organize it as a chunk of inline-text or properly defined function which sole purpose is to display the help. So no ill done to the poor guy who will maintain your script in 10 years.