Several years ago, I had a similiar problem, because the spreadsheed module I used at the time was not able to handle background colo(u)rs. My solution involved creating a macro in OpenOffice, and in the Perl script I had

system ('C:\Program Files (x86)\OpenOffice 4\program\scalc.exe', 
        '-headless', 
        'macro:///Standard.Module1.ConvertToHtml("' . $xlsx . '","' . 
+$file->canonpath . '")') if ! $file->exists;
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and using the resulting file. Sadly, I don't have the macro anymore, but I remember it was very short (some "open" and "save as"), similiar to simply open and hit "save".

I don't remember why I used OpenOffice instead of Excel. Perhaps it was easier to call the parameterized macro from Perl, but maybe not...