use strict;
use warnings;

use Tk;

my $top = new MainWindow;
$top->Button( -text => 'Start LongTime.exe', -command => \&longtime )
    ->pack;
my $diag = $top->Dialog( -text => 'Working...', -buttons => [] );

MainLoop;

sub longtime {
    $diag->after( 100,
        sub {
            system('longtime.exe');
            $diag->{selected_button} = 1
        } );
    $diag->Show();
}