If you are on Windows box, i'd highly recommend using Win32::OLE...
Here's a very good link: Tek-Tips
For WriteExcel formatting:
You need to first add the format:

 $footer = $excel->add_format(); $footer->set_font('Verdana'); $footer
+->set_color('Blue');
    $footer->set_size(9); $footer->set_bold(); $footer->set_top(); $fo
+oter->set_left();
    $footer->set_right(); $footer->set_bottom();
[download]

Something similar to the above.
Define this when the condition becomes true and then

$sheet->write($srow,$scol,'=FORMULA',$footer);
[download]