That's actually possible using existing horisontal scalability of HyperEstraier.
in reply to Re: Writing inverted index code in perl might be overkill
in thread Writing a Search Engine in Perl?
Just setup multiple servers which crawl separate parts of web. Setup search to search over all nodes at once.
Indexer can query search index to find out if some other indexer did crawl that page already (and optionally refresh content if needed). That way, you will have fresher pages with bigger number of incomming links (which you can count and use that also in page ranking - I hope that this idea doesn't violate Google patent).
I don't have pointer to perl solution for this (other than CPAN modules which make every problem 90% done). On the other hand, with current P2P architecture you can have multiple indexes (for e-mail, documents, etc.) and search over just some or all of them.