1. The documentation of the Tkx distro itself; it includes a Tkx::Tutorial.
2. The perl.tcltk newsgroup. It is archived here.
3. TkDocs. Has reference materials and tutorials. Perl examples are for Tkx, not Tk!
4. Tcl/Tk documentation - the horse's mouth.
5. Online documentation for ActiveState Tcl.

Also check out Tcl::pTk, which provides direct access to Tcl/Tk (that's right - Tcl) but via the Perl/Tk.pm syntax you know and love.