Heh ... so I was right ... there *is* a rational explanation ;-)
I also noticed that, in the original post, Errno.pm was reported as being in perl/site/lib - whereas I'm more used to seeing it in perl/lib. But I didn't pursue that aspect - mainly because I'm no longer sure just where ActiveState are placing the core modules.
Like you, I'm (now) thinking that the duplication may well be with Errno.pm, rather than Config.pm/Config_heavy.pl.