my $format_page_num = $workbook->addformat();
$format_page_num->set_size(10);
$format_page_num->set_font('courier');
$format_page_num->set_align('right');
$format_page_num->set_merge();

#merge cells and write page_num 
$worksheet->merge_cells(0, 0, 0, $last_column);
$worksheet->write(0, 0, $page_number, $format_page_num);
for $c (1 .. $last_column) {
      $worksheet->write_blank(0, $c, $format_page_num);
}