When you copy a site (test/devel site to production) the absolute path of PEAR packages is not the same so the module does not work, and your cron fail silently.
You can and must use a path relative to the base of your site (like sites/some/modules/pearwiki_filter/). It works well, so there is nothing wrong with the module, but the documentation that might tell "use a path relative to the base directory of your site".