The Perl developers should then talk to each other, look for a consensus and make this the way it is done.
Agreed. And since there are already multiple competing modules to handle type constraints and OO in general any Perl programmer could happily ignore anything added to the language and continue to use their existing favourite module because TIMTOWTDI.
having more than one in a project, which is inevitable for bigger projects, hurts.
Ain't that the truth. It certainly looks like that particular horse has long since bolted, unfortunately.