First "I am not a lawyer". Having said that, I have read the copyright law as well as I could. There seem to be several considerations that affect the seriousness of an alleged copyright infringement. One is the issue of 'fair use', which is a vague and often ambiguous term. Another consideration is the use of the material - whether for profit, or for educational purposes. A third issue is the harm, if any, done to the copyright holder. In the case of Perl Monks, the use is educational. Chances are good that the original author never expected to make any money on whatever he posted on the Internet. And it would be difficult to show that posting the material here caused the person any harm. One thing that appears clear is that anyone who publishes copyrighted material is at least theoretically liable, whether they knew of the alleged violation or not. The logic of this escapes me, but as I said, IANAL. I think Perl Monks will probably be OK in spite of the occasional plagiarized post. Perhaps the question isn't so much what we can get away with legally, but what is the right thing to do - and that, as we've seen, is a matter of considerable controversy.
More information can be found at copyright.gov.