when you first use the CPAN shell it asks you a couple of configuration questions. One of the questions asks for 'makepl' arguments (don't remember the specific term), if you enter 'PREFIX=/home/user/perl' (change the path to reflect the location you want your stuff installed in), the CPAN shell (for that user) will always install in /home/user/perl (if the modules take the PREFIX directive).

You can change your CPAN shell config by editting it's configfile (usually in ~/.cpan/CPAN/MyConfig.pm)