Did the authors pay attention to the copyrights on the perldoc documentation? From perltoc in the 5.005_03 version:

       Bundled Distributions

       When included as part of the Standard Version of Perl, or
       as part of its complete documentation whether printed or
       otherwise, this work may be distributed only under the
       terms of Perl's Artistic License.  Any distribution of
       this file or derivatives thereof outside of that package
       require that special arrangements be made with copyright
       holder.

       Irrespective of its distribution, all code examples in
       these files are hereby placed into the public domain.  You
       are permitted and encouraged to use this code in your own
       programs for fun or for profit as you see fit.  A simple
       comment in the code giving credit would be courteous but
       is not required.
[download]

So feel free to borrow from the code examples in the documentation, but you are not free to just slap together the documentation, edit the content lightly, and call it a book.

(Incidentally this site would be fine since it is attempting to display the complete documentation.)

(Yes, I know there is irony in Tom Christiansen caring about this, but let's not talk about that in public.)