Here is a Win32::OLE example:

use Win32::OLE;

# If Excel is already running it uses the copy in memory
eval {$ex = Win32::OLE->GetActiveObject('Excel.Application')};
if ($@) {
   $ex = Win32::OLE->new('Excel.Application', sub {$_[0]->Quit;})
          or die;
}

# open an existing workbook
$book = $ex->Workbooks->Open( 'yourdoc.xls' );

#write to a cell
$book->Worksheets(1)->Cells(1,1)->Value = "whatever";

#save and exit
$book->Save;
undef $book;
undef $ex;
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This should get you pointed in the right direction, if you are looking for more documentation check out some of the previous issue of "The Perl Journal" I know there was some good write up in there, and also O'Reillys Perl Resource Kit Win32 Edition has more then enough to tackle this type of job.

check this review as well Spreadsheet::WriteExcel