my     $MamaGUI = new MainWindow( -title => "Mein Programm");

# Größe des Fensters:
my      $windowHeight       = "600";
my      $windowWidth        = "800";

# Bildschirmgröße holen:
my      $screenHeight       = $MamaGUI->screenheight;
my      $screenWidth        = $MamaGUI->screenwidth;

# MamaGUI zentrieren:
$MamaGUI->geometry($windowWidth."x".$windowHeight);
$MamaGUI->geometry("+" .
                   int($screenWidth/2 - $windowWidth/2) .
                   "+" .
                   int($screenHeight/2 - $windowHeight/2)
                  );

# minimale Größe festlegen:
$MamaGUI->minsize( 400, 300);