See perldoc CPAN for your question on autobundle. make install copies all the files required by the module to their final locations (e.g. /usr/share/perl/5.8.6). To upgrade to perl5.8.5 you can try:

cpan> install NWCLARK/perl-5.8.5.tar.gz
[download]

The other approach to upgrade perl through CPAN is to force the install of some core module:

cpan> force install Benchmark
[download]

but this upgrades to the version that CPAN considers the "current" Perl version, which in this case is 5.8.6. Also you may want to consider the advice on upgrading here.