I don't know how you show your message, but the code below updates the title bar of the main window to "show progress"

use warnings;
use strict;
use Tk;

my $main= MainWindow->new (-title => "Test window");

$main->update ();
sleep (2);

$main->title ("Message");
$main->update ();
sleep (2);

$main->title ("Startup done");
MainLoop;
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