Do I understand well that if one sub wants to access another sub,
they could be both written as private  as long  as they are written in
the same module?
Then I shall reorganize my modules to put in the same modules the
subs that are used in the same process and not needed anywhere else.

If I am right, don't bother to reply :-)

When you write:
"For variables (attributes, or whatever you like to call them) in
modules, consider always keeping them private and writing access
functions and subs to get and set them if they need to be accessed
from outside the module."
I muss confess that I do not quite understand as well, probably
because I did not meet the problem, or had the need for it.
But I'll save your message and come back to it when I need it.

At the stage where I am, I would never ignore the experienced
guidelines from someone generous enough to reply to my questions!