If you thought that was a joke, you're still missing the point.
I suffer no illusion that their sales numbers have any correlation to the popularity or efficacy of a technology. Their Ruby book sales crashed in 2007, for example.
Who is writing new projects in Perl 5?
I know of many, many new projects written in Perl 5. So far this year I've worked on three and have another later in the year.
So O'Reilly can sell books about it.*
"Despite the existence of Rails, Perl 5 is still more widely used than Ruby."
Who is writing new projects in Perl 5? Ruby will also have a lot of legacy code in five years.
* If you thought that was a joke, you're still missing the point.