I'm not sure what distribution of Perl that you're using. I'd recommend taking a look at Strawberry Perl, which includes the compiler tools needed to allow you to immediately install modules from CPAN.
Strawberry Perl 5.16.1 32-bit already includes the Math::Pari and Net::SSH2 modules. Strawberry Perl 5.16.1 64-bit already includes the Net::SSH2 module.
Rather than trying to "fix" your current Perl install, I think that it might be quicker and less painful to move to Strawberry Perl, which already has the module that you're wanting installed and has the compiler tools to allow module installation directly from CPAN.