IIRC, PERL5LIB was introduced around the time perl5 say the light (hence the name), and it allowed people to have both a perl4 and a perl5 installation, using PERLLIB for perl4, and PERL5LIB for perl5.
So, I guess your complaint is "CPAN doesn't work with perl4" ;-)
Comment on Re: PERLLIB vs PERL5LIB: not quite the same
I think it's for that reason (i.e. being a Perl 4 thingy) that PERLLIB is tagged as obsolete in many people's heads (including mine) — for example see Motivation behind PERL5LIB and PERLLIB?.
I'm wondering, however, has it ever been officially deprecated for Perl 5? And if so, shouldn't there be a recommendation in perlrun etc. to use PERL5LIB by default these days?