Possibly the most convenient way to have multiple concurent versions of Perl available is to create a symlink so that /usr/bin/perl588 points to your Perl 5.8.8 install /new_perl5.8.8_dir/usr/local/bin/perl.

ln -s /new_perl5.8.8_dir/usr/local/bin/perl /usr/bin/perl588

Then you just need to do

#!/usr/bin/perl588

to use it in a script and

perl588 Makefile.PL
[download]

to do the install of CGI::Session into the 5.8.8 version of Perl. All your 5.8.0 stuff remains unchanged so nothing old should break.