In this sense, I feel that thought could be given to a natural and agile way (built into the structure of CPAN) of branching or evolving a given module, a way that automatically took care of authorship, copyright and and other important formalities.
I don't think this should be build in the structure of CPAN. The ability to fork is a licensing
issue - most open source licenses allows you to fork. It's totally indepent of the existance of the module on CPAN.
That is, so that every bit of work by any author (no matter the level of it's original author's involvement after it is released) would be completely shared with the other authors and users.
That's not the task of CPAN. CPAN should not dictate what rights authors must be willing to give up for distributing a module via CPAN. The only restrictions CPAN should put on authors are those that keep CPAN from doing its work, as such, the restrictions for authors is that their work should be freely distributable.
Adding the requirement that outsiders may take over authorship and copyright of a module isn't a good thing in my opinion. You'll lose people contributing code to CPAN. Not to mention all the resources you have to pour in to work out the legal details of this.