Are $mw->Busy; at the beginning and $mw->Unbusy; at the end of the rearrangement enough?

Thanks for that suggestion.
However, perhaps not.
I just tried by adding $mw->Busy but the GUI was updated.
Should this have happened?

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

my $config;

my $mw = MainWindow->new(-title=>"Demo");
$mw->withdraw;

get_config($mw);
MainLoop;

sub setup_mw{
 my $label = $mw->Label(-text=> $config)->pack();
 $mw->deiconify;
 $mw->raise;
}

sub get_config {
   my $parent = shift;

   #collect your config here
   $config = 'hoohah';
   
   my $win  = $parent->Toplevel(-title=>'config window');
   my $Bttn = $win->Button(
       -text=>"I'm done configuring, so press me",
       -command=> sub{
         $win->destroy;
     setup_mw();
       
       }
   )->pack( );
}
__END__
[download]

I would prefer not to minimise the GUI.
I am using the grid method to place the widgets.
Is there a way I can set the grid values but then make
these effective (so that the new form of the GUI
is obtained) at one go?

I use pack almost exclusively, but have you read "perldoc Tk::grid" ? There is gridForget and gridPropagate, etc, so you should be able to find a way.

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I'm not really a human, but I play one on earth. Cogito ergo sum a bum