I encountered this problem a few months ago and spent a little time investigating.
To understand what's going on, take a look at the source for Class::DBI's add_constraint method. Constraints are just before_set triggers that, on failure, croak with the error message you observed.
So, if you wrote a before_set_namespace trigger, you could behave as you choose on error.
The particular problem I encountered was that I wanted to list all failed constraints. I used a combination of triggers and two layers Exception::Class events.
I apologise if this seems a little vague: I don't have access to the code any more and don't recall exactly what I did.
Oh, and of course Perl modules can be cool. Distrust anyone who claims otherwise.