Where's Protocol-Modbus?

On search.cpan.org I can find Protocol::Modbus and it looks like a typical
module.  I can download it, it's passed tests (including my matching Perl
version and platform), etc.  But using "cpan", it can't find it, unlike
many other modules I've retrieved via "cpan".  I looked in the CPAN FAQ
but it didn't mention this situation.  What's up?