[en] Adding Perl Book (and removing Perl 6 duplicated)

free-programming-books.md

@@ -1221,12 +1221,12 @@ See also [TeX](#tex)
 * [Perl & LWP](http://lwp.interglacial.com/index.html)
 * [Perl for the Web](http://www.globalspin.com/thebook/)
 * [Perl Free Online EBooks](http://linkmingle.com/list/13-plus-List-of-Free-Great-Perl-Books-available-Online-freebooksandarticles) (meta-list)
+* [Perl Training Australia - Course Notes](http://perltraining.com.au/notes.html)
 * [Practical mod_perl](http://modperlbook.org/) - Stas Bekman, Eric Cholet
 * [Plack Handbook](http://handbook.plackperl.org/)
 * [SDL::Manual Writing Games in Perl](https://github.com/PerlGameDev/SDL_Manual)
 * [The Mason Book](http://www.masonbook.com/book/)
 * [The PDL Book](http://sourceforge.net/projects/pdl/files/PDL_2013/PDL-Book/PDL-Book-20130322.pdf/download) (PDF)
-* [Using Perl 6 - a Perl 6 book in development](https://github.com/perl6/book)
 * [Web Client Programming with Perl](http://oreilly.com/openbook/webclient/)