Thanks. A little fettling gets the job done. Win32::OLE makes a whole lot of function calls look like empty hash references. But an empty hash ref is not undefined and can be tested for.

 $docprops =  $doc->CustomDocumentProperties();
 unless (defined ($doc->CustomDocumentProperties($property)))
 {
    $docprops->Invoke('Add', {
                               Name => $property,
                               LinkToContent => 0,
                               Type => 4, #msoPropertyTypeString 
                               #strictures to work around still
                               Value  => $value,
                              });
  }
  $doc->CustomDocumentProperties($property)->{Value} = $value;
[download]

It looks a little weird in the debugger, as the fact that the value is set is hidden for a little while (value is ''). Suspect it's a debugger/tie/Windows interaction.

Thoglette.