The Tcl::Tk submodule of the Tcl module gives access to
       the Tk library.  It does this by creating a Tcl
       interpreter object (using the Tcl extension) and binding
       in all of Tk into the interpreter (in the same way that
       wish or other Tcl/Tk applications do).

       Access to the Tcl and Tcl::Tk extensions

       To get access to the Tcl and Tcl::Tk extensions, put the
       commands
           require Tcl;
           use Tcl::Tk;

       near the top of your program. The Tcl extension does not
       alter your namespace at all (hence the "require"). The
       Tcl::Tk extension imports the widget and other Tk commands
       into your namespace (hence the "use").