Have you head of Carp::Croak()?

Have you read the word optional? Why are you ignoring all other of my points? If you don't have an actual argument but just want to be right then say so, and I'll just shut up.

If you think that $foo eq 'Something' and $foo->isa( 'Something' ) are vastly different...you should be concerned.

I disagree. Strongly.

The bottom line is, either technique requires programmer discipline. If you really used 'MyError' for your exception namespace, you are just as vulnerable to someone else choosing that badly chosen namespace for their exceptions, as I would be if I used a non-specific formating convention for my error messages.

I disagree. First off, everything in programming requires discipline. Cooking and exception handling both require discipline, that doesn't make them identical. The big difference is that class namespaces are designed to be namespaces. Strings aren't. You can assume part of a string is a namespace, but you can never be sure. A class is always a namespace. It's designed for the job you want to cram into a string.

You also say yourself why strings are less reliable for error handling: It's depending on your error message formatting, *plus* the handling of errors by a type you want to extract from a free-form value that might have come from anywhere. With objects, all you need to check is the type. It doesn't matter then how I format my errors, how you format you errors, how I format your errors and how you format my errors.

If programmers adhere to a standard for their errors or exceptions, the problems are alleviated.

With object oriented exceptions, there are the enforced standards of objects. Specifically that data and package are separate pieces of information that I can access reliably. And as I've said earlier, if you're the only programmer working the code, and never upgrade your modules, and know all formats of errors all other authors used, then yes, it might "work."

But exceptions are not a magic bullet that will fix badly architected code.

Nothing will do that, but that's not an argument on the issue. I'd argue that inspecting user error messages for error type determination is bad code.