$_app_object->ActiveSheet->PivotTables('PivotTable1')->PivotFields
+('a')->{Subtotals} = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
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I think the error is earlier. If you add the following lines to your code, it will tell you where:

use Win32::OLE::Const 'Microsoft Excel'; # instead of the constants

Win32::OLE->Option( Warn => 3 );
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something seems to be wrong at CreatePivotTable

$_app_object->ActiveWorkbook->PivotCaches->Add({SourceType => xlDa
+tabase, SourceData => 'Sheet1!R1C1:R4C3'})->CreatePivotTable({TableDe
+stination => 'Sheet1!R8C3', TableName => 'PivotTable1', DefaultVersio
+n => xlPivotTableVersion10});
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Best regards,
perl -e "s>>*F>e=>y)\*martinF)stronat)=>print,print v8.8.8.32.11.32"

Thanks for taking the time to help! I added the lines you suggested. There was one warning that I've fixed, but now I still get the same problem. I have made no changes to the other line you were suspecting. The error is

OLE exception from "Microsoft Office Excel":

Unable to set the Subtotals property of the PivotField class

Win32::OLE(0.1704) error 0x80020009: "Exception occurred"
    in PROPERTYPUT "Subtotals" at c:\bin\tmp.pl line 54
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The pivot table code is reduced to

$_app_object->ActiveWorkbook->PivotCaches->Add({SourceType => xlDataba
+se, SourceData => 'Sheet1!R1C1:R4C3'})->CreatePivotTable({TableDestin
+ation => 'Sheet1!R8C3', TableName => 'PivotTable1', DefaultVersion =>
+ xlPivotTableVersion10});
   $_app_object->ActiveSheet->PivotTables('PivotTable1')->AddDataField
+($_app_object->ActiveSheet->PivotTables('PivotTable1')->PivotFields('
+a') );
   
   $_app_object->ActiveSheet->PivotTables('PivotTable1')->PivotFields(
+'a')->{Orientation} = xlRowField;
   $_app_object->ActiveSheet->PivotTables('PivotTable1')->PivotFields(
+'a')->{Position}    = 1;

   $_app_object->ActiveSheet->PivotTables('PivotTable1')->PivotFields(
+'b')->{Orientation} = xlRowField;
   $_app_object->ActiveSheet->PivotTables('PivotTable1')->PivotFields(
+'b')->{Position}    = 2;

   $_app_object->ActiveSheet->PivotTables('PivotTable1')->PivotFields(
+'a')->{Subtotals} = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
[download]