When you program in Tk, you have to change your mindset to think in terms of event-driven programming.

Here's an example of how you could use the Tk "repeat" function, to execute a command every second:

# 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 millisecon
+ds)
my $loopStop = 0;    # A pointer to this will be passed to the subrout
+ine
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;
}
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Note that you don't need to sleep in the subroutine; that would only slow down the entire GUI, as jdtoronto pointed out.

When the value of $hour (however you are calculating it) is equal to $exeTime, the variable pointed to by $ploopStop is set to 1, so on subsequent calls, the logicthe documentation for Tk::repeat to see how to cancel an event loop.