There are really two issues here. The one everyone seems to be discussing are the moral and ethical ramifications and what to do about the palgiarism. What everyone seems to be avoiding are the legal issues of copyright infringement. Slapping "plagiarized" onto these nodes does not negate the copyright issues. And just downvoting these nodes most certainly does not.
Even if Perl Monks is not legally bound to remove the offending nodes (until asked to by the copyright holder) it certainly is the ethical thing to do. At least that's the kind of response I would expect from a reputable site if they discovered they were hosting illegal copies of my computer artwork (or Perl poetry if I ever got around to writing some).
With that said, there is merit to keeping an archive of these nodes for future reference. A compromise might be to assign a special plagiarized attribute to the offending nodes and then only allow Friars on up to search for these (and searching for that attribute would be off by default of course).