You can withdraw the $mw while reconfiguring it, and pop up a "please wait" toplevel, something like below. Otherwise, you can packForget all the widgets, rebuild them without packing, then pack them all at once.

#!/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__
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