You need to compile and install this by hand. Luckily that is easier than you might think. I will assume a Unix system of some sort, and assume that you are not in a mod_perl environment.

You need to download it, tar -xvzf perl-5.8.7.tar.gz it, then cd into the directory that was created and run ./Configure (the defaults will be OK on everything except where to install it, you want it installed to a local directory), then make, make test and finally make install.

I would personally advise doing this as an unprivileged user so you don't accidentally overwrite any system libraries that might exist.

That should install it to whatever directory you specified. This installation is completely independent of your existing Perl. So the next thing you need to do is /path/to/new/version/bin/perl -MCPAN -e shell and go about installing all of your CPAN dependencies. Then run your application against this version of Perl.

If modifying your application to run against the new location of Perl is too much work, as a hack you can (as root) move your existing /usr/bin/perl then ln /path/to/new/version/bin/perl /usr/bin/perl, test your application, then remove your symlink and copy the old version of Perl back when you're done.

If you are on a non-Unix like OS (eg Windows), then you may not have all of the tools for the above to work. If so then you'll need to manually locate a copy of an older version of Perl for your platform and follow its instructions. Likewise if you're using mod_perl then you'll need to compile Apache with mod_perl with an older version of Perl.