Anyway isn't there a way to run su -c "make install" instead?

The answer is right there in the CPAN documentation: http://search.cpan.org/~andk/CPAN-1.9301/lib/CPAN.pm
search for sudo. set the mentioned variables make_install_make_command and mbuild_install_build_command to the values which are suggested there and the install will be called with sudo.