# Create a new main window object called $mw
my $mw = new MainWindow();

# ... other GUI setup here

# Call repeat to execute your subroutine every second (1000 milliseconds)
my $loopStop = 0;    # A pointer to this will be passed to the subroutine
my $hour;            # However this is calculated ...
$mw->repeat(1000 => [ \&CheckTime, \$loopStop, $hour ]);
MainLoop();          # The MainLoop goes last


# And here's the subroutine                                                                           
sub CheckTime {
    my ($ploopStop, $hour) = @_;
    $exeTime=18;
    if ($$ploopStop == 0) {
        my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst)
            = localtime(time);
                                                                                
        $ZWIDGETS{'Text1'} -> insert('end',"Waiting......\n");
        if ($hour == $exeTime) {
            $ploopStop = 1;
            $ZWIDGETS{'Text1'}->insert('end', "Starting.....\n");
        }
    }
    return $hour;
}