The short install procedure is described in README.  Here is the long one:

If you have Net::FTP module installed (type

	perl -MNet::FTP -le "print 'OK'"

to check, and, say

	perl -MCPAN -e 'install Net::FTP'

to install it if you do not have it), then Makefile.PL may fetch and
configure PARI for you, so you can skip steps a,b,c,e in the description
below.  You need to manually apply patches *after doing*

	perl Makefile.PL

if the PARI distribution is 2.0.11/2.0.12, maybe later too.

If you have a not very old perl (5.003 should do), then

  a) Get the latest PARI distribution from
	ftp://megrez.math.u-bordeaux.fr/pub/pari/unix/

  b) uncompress the distribution and run 'Configure' script
	sh ./Configure

  c) It is not required to make PARI, but you can do it nevertheless:
	make test
     in toplevel PARI directory;

  d) uncompress the Math::Pari archive into a separate directory (for
     best results make it a subdirectory of PARI's root directory);

  e) Apply patches from patches subdirectory of Math::Pari (patches
     are made for PARI 2.0.11.beta, your mileage may vary);

	cd pari-2.0.11-beta          # or whatever it is
	patch -p1 < ../patches/diff_pari_gnuplot_aa
	patch -p1 < ../patches/patches_round1
	patch -p1 < ../patches/diff_pari_fixed_interfaces_011 
	patch -p1 < ../patches/diff_pari_highlevel_hash_011a
	patch -p1 < ../patches/diff_pari_ret_proto_2011

  f) _Optionally_ find the architecture of your processor (if it is not
     known/supported and recognized automatically, the slower 'port' code
     without assembler should be used).  Recognized "processors" are 

	sun3 sparcv7 sparcv8micro sparcv8super alpha hppa i386 port

     the current version should autorecognize them in most situations;

  g) do

	perl Makefile.PL 
		or
	perl Makefile.PL machine=hppa
     then

	make
	make test

     (expect several failures from t/*_will_fail.t, but other tests should
     pass). 

  h) Now

	make install

     should make this extension available to the perl programs.

