c'est bon j'ai trouvé, il suffit d'utiliser la méthode write_blank(...) avec le meme format d'ecriture que précédemment.
# Create a border format
$border1 =& $workbook->addformat();
$border1->set_color('white');
$border1->set_bold();
$border1->set_size(12);
$border1->set_pattern(0x1);
$border1->set_fg_color('green');
$border1->set_border_color('yellow');
$border1->set_top(6);
$border1->set_bottom(6);
$border1->set_left(6);
$border1->set_align('center');
$border1->set_align('vcenter');
$border1->set_merge(); # This is the key feature
$headings = array('Statistiques concernant : Datfac, Datbaan, Colis', '');
$worksh1->write_row(2, 0, $headings, $border1);
$worksh1->write_blank(2,1,$border1);
$worksh1->write_blank(2,2,$border1);