Following your wise advices, I have learnt to turn on various traces in modperl (StatINC), and the problem appears now on a new light : in some cases
require 'gencalendar.pl';
does not load it and gives :
Apache::StatINC: Can't locate gencalendar.pl
but in some cases it success locate it...
I thank you all, serach more and start a new node on this somehow different problem if I do not find.