Actually, IIRC, for quite a while, 5.9.x *had* err, and it was culled not to long before 5.10 came out due to some backwards compatability issues noone seemed to be able to resolve.
Comment on Re^4: RFC: Defined-Or for before Perl 5.10
I'm actually quite happy that the 5.8.x series is out of maint now. Maintaining those patches was quite a lot of work.
Note that those patches still add the err keyword to perl.
The disadvantage of those patches over the proposal of a module/function, is that it is inside the perl binary, and all has to be recompiled from scratch.