Looking at your original post, I'm a little confused as to why you need to edit a cell. If you use Spreadsheet::ParseExcel::SaveParser you load in your existing spreadsheet then create your new worksheet:

$oBook->AddWorksheet('February');
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To this new blank worksheet first add the cells from the prior month's worksheet that are the same in the new month's worksheet for example:

my $oOld_Sheet = $oBook->{Worksheet}[0];
for (my $row = 0; $row <= 4; $row++) {
    my $oCell = $oOld_Sheet->{Cells}[$row][0];
    if (defined $oCell->{Val}) {
        $oBook->AddCell(1, $row, 0, $oCell->{Val}, $oCell);
     }
 }
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Adding the sales for the new month to the new sheet using the AddCell method as well. Finally save the workbook under a new name using the SaveAs method.

If you really need to edit a cell then Win32::OLE as InfiniteSilence and davidrw mentioned is probably what you will have to use, also if the worksheet you are adding contains formulas.