I continued the investigation...
I noticed that C::B version 4719 has been installed by Yum (packet manager for Fedora) on December the 25th, precisely the moment where the problem occurred.
I downgraded C::B to version 3816 and the problem has now disappeared.
But that is not as simple as it seems : I first tried to downgrade for a version (I've forgotten the number) that was installed on my computer before the 4719 and at that time there was no problem.
But the bug was still present after the downgrade.
Well, I may be happy with version 3816. But now, I have no console for standard output/input in debug mode ...
So, I still cannot work on my project.
Thanks for any information tha can help
Arnaud