Hi! I think this is my first contribution hehe... I Got this code from the php cookbook, and converted it to Perl. It worked for me :)

    my $back_pixbuf =  Gtk2::Gdk::Pixbuf->new_from_file("apple.png");
    my ($pixmap,$mask) = $back_pixbuf->render_pixmap_and_mask(255);
    my $style = $window->get_style();
    $style=$style->copy();
    $style->bg_pixmap("normal",$pixmap);
    $window->set_style($style);
[download]

NOTE: apple.png NOT INCLUDED.