There are many on google, but a few good ones are
Tk tutorial, and
uwinnipeg Tk tutorial. Also, there are 2 books available online, the oldest is Learning Perl/Tk, and the best is Mastering Perl/Tk. You can find sample chapters online, and the "Perl CD Bookshelf Version 3.0" ( an older version of the bookshelf) has 6 Perl books on it including Mastering Perl/Tk. A quick look at Ebay, has one for $22.50, pretty good for 6 Perl books. I don't know the legality of this site, but look at
Mastering Perl/Tk