Just to broaden your horizons a bit, Perl/Gtk2 has a neat way to handle it. You can set an entrie frame or widget to visible(0) or visible(1). That way you can multiple widgets located in one screen spot, and choose which one to show. packForgetting is clumsy compared to that.

For an example of progressbar, see Tk-with-worker-threads. I made my own progressbar in that, because the regular Tk::Progressbar widget leaks memory on reuse, probably because it uses Photo objects internally, that leave refcounts behind. But the following works pretty well.

#!/usr/bin/perl
use strict;
use Tk;
use Tk::ProgressBar;

#dosn't leak

my $mw=MainWindow->new;
$mw->geometry('+100+100');

my $tframe = $mw->Frame->pack();

my $pro_on = 0;
my $count = 0;

my $label = $tframe->Label(
           -text => 'Show Progress',
          )->pack(-side => 'left', -padx => 5);


my $cb = $tframe->Checkbutton(
            -activebackground => "white",
            -variable => \$pro_on,             
            -command  => [ \&display_pro ]
          )->pack(-side => 'left');

my $bframe = $mw->Frame->pack();

my $pb = $bframe->ProgressBar(
   -height      => 0,
   -width       => 20,
   -length      => 100,
   -colors=>[0,'blue'],
   -blocks=>100,
   -variable => \$count,
   )->pack();

# need to pack then unpack them to get right window size
my @w = $bframe->packSlaves;
foreach (@w) { $_->packForget; }

MainLoop;

sub display_pro {
   print $pro_on,"\n";
    
   if($pro_on){
      $count  = 0;
      $bframe->pack();    
      $pb->pack();
#      $mw->packPropagate;
#      $mw->update;
      show_progress();
   }else{
   
     my @w = $bframe->packSlaves;
     foreach (@w) { $_->packForget; }
     $bframe->packForget;
   
   }
}

sub show_progress{
  my $timer;
  $timer = $mw->repeat(100,sub{
       $count++;
       $pb->value($count);
       $mw->update;
       if($count == 100){$timer->cancel}
     });
}
__END__
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