There is a thread on PerlMonks that starts with How to prevent CPAN from updating Perl [perlmonks.org]. I have not followed the instructions myself, but Randal Schwartz recommended updating CPAN.pm in order to avoid the update to Perl 5.6.1. Seems like a reasonable bet to work for Perl 5.8.0.
PerlMonks Says How to Stop a CPAN Perl Upgrade (Score:1)
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Dave Aiello
Chatham Township Data Corporation [ctdata.com]