ppm install http://theoryx5.uwinnipeg.ca/ppmpackages/XML-LibXML.ppd

Wrong link - that's the perl-5.6.x repository.
For your version of perl (5.8.2) you want:

ppm install http://theoryx5.uwinnipeg.ca/ppms/XML-LibXML.ppd
[download]

Cheers,
Rob