IMHO the ... doesn't make much sense in Perl 5.

In Perl 6 it is actually not the same as a fail('Unimplemented') - classes that only consist of ... are "stubbed", and you can declare them later on without error:

# this is fine:

class A { ... }
# other code here
class A {
    # actually implement A here
}


# this dies, because class B is re-declared

class B { fail('Unimplemented') }
# other code here
class A { # <-- ERROR
   # whatever
}
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Maybe someday the yada-yada-yada term will throw an error in Perl 5 that's somehow distinct from other errors (like a different exception class), but until that time I don't really see the use.