You can make the whole window busy, which will do the right thing automatically:

$mw->Busy( '-recurse' => 1 );
... do some other stuff here ...
$mw->Unbusy( '-recurse' => 1);
[download]

Or you can just change the cursor, although this method doesn't prevent people from trying to interact with your application while it is busy, which the above does:

$mw->configure(-cursor => 'watch');
... do some stuff here ...
$mw->configure(-cursor => undef);
[download]

Thanks jasonk!