If you are using the latest version of Spreadsheet::ParseExcel then the code giving the warning is as follows:

        ...
        # Print Gridlines
        if ( $oWkS->{PrintGrid} == 1 ) {
            $oWrS->hide_gridlines(0);
        }
        ...
[download]

The PrintGrid option should be set to a default value on creation but perhaps there is a bug somewhere.

If you like you can submit a bug report here. Follow the instructions on that page and submit the Excel file that you are having issues with. It won't be possible to debug it without the Excel file.

Alternatively, you can just ignore the warning (mentally or programmatically) if it doesn't affect the end result.

--
John.