According to Win32::GUI::ToolTip:

Tooltip controls are probably one of the most unintuitave of the Win32 controls when you first come accross them. A Tooltip control is a single window that supports one or more 'tools'. A tool is a window, or an area of a window that when the mouse hovers over, the tooltip window is displayed. The Tooltip is always a top level window (so don't try adding the WS_CHILD window style), and is typically owned by the top level window of your application/dialog.

Create a tooltip window:

  my $tt = Win32::GUI::Tooltip->new(
    $main_window,
  );
[download]

Add a tool to the tooltip:

  $tt->AddTool(
    -window => $main_window,
    -text   => "Text that pops up",
  );
[download]

and hover the mouse over an area of your main window.

I just tested that you can call AddTool again to change tip message. You need to specify your button with $MAIN->Load_3, to get tooltip over your button.

-- Roman