Your hexa hwnd is not the good one :)
Here your script with a little update
Have a nice day :)
use Tk;
use Tk::Balloon;
use Win32::API;
use vars qw/$Button/;
my $mw = new MainWindow;
$id_mw = $mw->id;
$mw->Frame(-width=>200, height =>25);
$mw->update;
Win32::API->new("user32","SetWindowPos",
[qw(N N N N N N N)],'N')->Call(hex($id_mw),-1,0,0,0,0,3);
$Button=$mw->Button(-text => "Button",
-command => sub {
&do_something;
})->pack();
my $balloon = $mw -> Balloon(-balloonposition => 'mouse',
-initwait => 10, -bg => 'white');
$balloon -> attach($Button, -balloonmsg => "Button");
MainLoop;
sub do_something {
#sleep(10);
}