Book sales are an unreliable indicator of "popularity" in so far as the share of Perl users may be more likely to find their information via other means. People buy books to help them learn a language. Perhaps Perl programmers, as a group, are more erudite when it comes to programming than users of, say, Visual Basic.
I'd also think that the /type/ of books being bought would be a factor here. Are they introductory texts, reference books, or cookbooks?
Finally, "popularity" is only partly the question. Deployment is important, too. If Yahoo's, eBay's, and other large companies' infrastructures are built on Perl, it's highly unlikely the language will "fall out of favor."
Obviously there are many flaws in using book sales as an indication of Perl's relevance.