Make sure that you also specify the proper prefix when installing modules, or you'll get EACCES (permission denied) when you try to install the files. A good way to ensure that you can't install files in your vendor's Perl directory is to install the other Perl's modules as a user who doesn't have write permissions to the vendor's Perl directory. I do this on my machine, and it works quite well.

Also, don't be an idiot like yours truly and forget to set PERL5LIB before installing modules with CPAN / CPANPLUS. If you do, you lock yourself into an infinite loop because the shell will repeatedly install modules but it won't be able to see them. :)