In my case, cpanm failed because pari-2.1.7.tgz has been moved to a sub-directory. I also had to drop sudo as this will be installed to ~/perl5. So here are the revised instructions:

Run: cpanm --look Math::Pari

download ftp://megrez.math.u-bordeaux.fr/pub/pari/unix/OLD/2.1/pari-2.1.7.tgz manually into that directory, complete the installation by running:

tar -xvf pari-2.1.7.tgz
perl Makefile.PL
make
make test
make install
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