in reply to Tk how-to?

Your direct question was already answered, I only want to note to you that a very similar to perl/Tk module Tcl::Tk is a bit harder to use at the beginning, but a much more flexible and capable, so, if you're starting, better to use it.
There were some show-stoppers in perl/Tk that made me move to Tcl::Tk.
(I am one of a maintainer of it, BTW)

Best regards,
Courage, the Cowardly Dog