I assume the publishers and some active authors check the internet regularly for such illegal copies, and then contact the providers, and ask them to remove the illegal content (and if necessary threaten them with legal action). Therefore a lot of those websites with illegal content come and go. Some of those websites are online for just a couple of hours a day, a couple of days a week.
Interested people better buy books on CD, like the O'Reilly Perl CD Bookshelf or pay for online available books, like the Safari Bookshelf Perl Books. That's the newest version, and the authors get their work paid as well.