FWIW, try giving the full path to perl. For example, on my system the main perl is /usr/bin/perl. If I want to use ActivePerl, then I have to use the full path /opt/ActivePerl-5.14/bin/perl. To use cpan, then I would give the path to cpan /opt/ActivePerl-5.14/bin/cpan to drop into a shell. Don't forget to use your version number:).