Thanks guys,

I finally did it.

$MAIN->AddButton(
     -name => 'Load_3',
     -text => "Load file",
     -size => [100, $BH],
     -pos =>[$LM, 223],
     -tabstop => 1,
);

my $TIP = $MAIN->AddTooltip();

$TIP->AddTool(-text => 'Load your file here', 
              -subclass => 1, 
              -window => $MAIN->Load_3);
$TIP->UpdateTipText($MAIN->Load_3,"Loading...");
[download]

The original -tip set in the creation of the button have been removed and changed using the AddTool above. Otherwise the UpdateTipText above won't work.