Tk the Perl module is an interface written by Nick Ing-Simmons to Tcl/Tk. Tcl/Tk is actually two products originally developed at the University of California Berkeley, now at Sun by John Ousterhout: the Tool Command Language and the Tool Kit. They are a versatile scripting language and a graphical interface development tool, respectively.
In brief Tk lets you build GUIs. If you build a Tk GUI with Perl it should run on just about anything Perl (and Perl Tk) runs on. By way of simple example it will almost certainly run on Win32 or Linux.