First make sure you have libgmp3, libgmp3-dev and libpari1 (or similar names for HPUX).
Then if you encounter any problems when installing Net::SSH::Perl from cpan prompt install each of prerequisite modules separately (either from cpan or from distribution - perl Makefile.PL, make, make test, make install).

Math::Pari installation may cause problems so read README and INSTALL of this module.