use strict;
use Tk;

my $mw = MainWindow->new();
my $button = $mw->Button(-text        => "do it",
                         -command     => \&callback  );
my $var = "Waiting ...";
my $lab = $mw->Label(-textvariable   => \$var);
$lab->pack();
$button->pack();

MainLoop;

sub callback {
   $var = "INFO: step 1, please wait ...\n";
   $lab->configure(-textvariable => \$var);
sleep 2;
   $var = "INFO: step 2, please wait ...\n";
   $lab->configure(-textvariable => \$var);
sleep 2;
}