I think the WaitBox module is buggy. I kept getting errors with the latest version of Tk. It's no wonder I hardly ever see WaitBox in use.

can't make ".waitbox" its own master at /usr/lib/perl5/5.8.6/i686-linu
+x-thread-multi/Tk/Submethods.pm line 37.
 at /usr/lib/perl5/site_perl/5.8.6/Tk/WaitBox.pm line 58
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If you are just looking for a dialog which grabs control, and prevents $mw from responding, try something like the following. There is also DialogBox which will allow you to place your Progressbar into it( I've not tested that combo , but it should work).

#!/usr/bin/perl
use warnings;
use strict;
use Tk;
use Tk::Dialog;

my $mw = MainWindow->new();
$mw->geometry("+100+100");
$mw->title("Main Window");
$mw->geometry("250x250");
$mw->bind("<Return>", \&do_dialog);

my $result = "Dialog Result Here";
$mw->Label(
 -textvariable => \$result,
 -relief => "sunken",
 -borderwidth => 1
 )->pack(
 -side => "bottom",
 -fill => "both",
 -padx => 5,
 -pady => 5,
 -ipadx => 3,
 -ipady => 3
 );

$mw->Button(
 -text => "Exit",
 -command => sub{ exit(0) },
 )->pack(
 -fill => "x",
 -padx => 5,
 -pady => 5,
 );

$mw->Button(
 -text => "Test Dialog",
 -command => \&do_dialog,
 )->pack(
 -expand => 1,
 -fill => "both",
 -padx => 5,
 -pady => 5,
 );

MainLoop;

exit(0);

sub do_dialog {

 my $dlg = $mw->Dialog(
  -title=>"Here is a question for you.",
  -buttons => ["Cancel", "No", "Yes"],
  -default_button => "No",
  -text => "Let's see what the result is..",
  -font => "Helvetica"
  );
 $result = "Result: " . $dlg->Show();
 print("got: $result\n");

 }
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