Due to the binary nature of the Excel file format you cannot use Spreadsheet::WriteExcel to write or add to an existing file.

However, you can use Spreadsheet::ParseExcel to read an Excel file and then use Spreadsheet::WriteExcel to rewrite it.

The Spreadsheet::ParseExcel package also contains a module called Spreadsheet::ParseExcel::SaveParser which will let you read and rewrite an Excel workbook. It is a combination of Spreadsheet::WriteExcel and Spreadsheet::ParseExcel.

Here is an example:

    #!/usr/bin/perl -w

    use strict;
    use Spreadsheet::ParseExcel::SaveParser;

    # Open an existing file with SaveParser
    my $parser   = new Spreadsheet::ParseExcel::SaveParser;
    my $template = $parser->Parse('template.xls');

    my $sheet    = 0;
    my $row      = 0;
    my $col      = 0;

    # Get a format from a cell
    my $format   = $template->{Worksheet}[$sheet]
                            ->{Cells}[$row][$col]
                            ->{FormatNo};


    # Write data to some cells
    $template->AddCell(0, $row,   $col,   1,     $format);
    $template->AddCell(0, $row+1, $col, "Hello", $format);


    # Add a new worksheet
    $template->AddWorksheet('New Data');


    # The SaveParser SaveAs() method returns a reference to a
    # Spreadsheet::WriteExcel object. If you wish you can then
    # use this to access any of the methods that aren't 
    # available from the SaveParser object. If you don't need
    # to do this just use SaveAs().
    #
    my $workbook;

    {
        # SaveAs generates a lot of harmless warnings about unset
        # Worksheet properties. You can ignore them if you wish. 
        local $^W = 0;

        # Rewrite the file or save as a new file
        $workbook = $template->SaveAs('newfile.xls');
    }

    # Use Spreadsheet::WriteExcel methods
    my $worksheet  = $workbook->sheets(0);

    $worksheet->write($row+2, $col, "World");


    __END__
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Note, however that this will only read and rewrite the features that Spreadsheet::ParseExcel and Spreadsheet::WriteExcel can handle so macros, graphs and some other features in the original Excel file will be lost.

--
John.