For historical reasons, most of the best Perl books are published by O'Reilly. I suspect this is at least partly because the Camel book (Programming Perl) was one of the really early Perl books and, because of who wrote it, also very good. Since that one was published by O'Reilly, which worked out very well for all concerned, some of the other people in the Perl community, when they wrote Perl books, knew about this and chose the same publisher. (O'Reilly is also well regarded in technical circles anyway, but IMO this is especially so in the Perl community.)

There _are_ other Perl books, of course. But the O'Reilly ones tend to be the better ones. For one thing they tend to be written by people whose names are already known in Perl circles, people who know Perl inside and out. (Although, some of those people have also gone on occasion with other publishers, e.g., Effective Perl Programming is published by Addison-Wesley, and MJD published with a company called Morgan Kaufmann, whoever they are, so O'Reilly doesn't have a total monopoly on good Perl books, just the majority of them.)