I apologise for 'replying' to my own question but this contains more details
that may help to understand what is happening.
I now realise that I have asked a similar question before but no Monk could help
However one Monk did want some more details. Here they are
I am using the following modules

use Win32;
use Win32::OLE;
use Win32::OLE::Const 'Microsoft Excel';
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and the following to open the workbook etc

     $excel = Win32::OLE->new('Excel.Application', 'Quit') or ($excel_
+res = 0, $ew_message = "Could not create excel object");
$workbook = $excel->Workbooks->Open($new_finnpower_full)  or ($workboo
+k_res = 0, $ew_message = "Could not open excel workbook");
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I believe that both of these worked since I could manually save the altered spreadsheet
and I did not get any errors from these statements.