The website you refer to is already gone. And no, such websites are not ligit. There are a lot of people who have digital versions of such books and they are actually not very hard to find. But they are infringing on copyrights and they do cost money to publishers and authors (arguably, one might say that someone who reads the digital illegal copy of an older version may be quite happy with it and than want to buy the legal edition of it, but people probably will just stick with the illegal version and even distribute it further).
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.