I can't give you much detail, but in our snippets collection from 5 or 6 years ago (Perl 5.6 days?) I found this: 
use strict;
use Win32::OLE qw(in);
use Win32::OLE::Const 'Microsoft Office';

my $Doc = Win32::OLE->GetObject('g:\scratch\test.doc');

if ( ( 0 + Win32::OLE::LastError() ) != 0 ) {
  $Doc->CustomDocumentProperties->Add(
    {
      Name          => 'A custom property',
      LinkToContent => 0,
      Type          => msoPropertyTypeString,
      Value         => "Whatever"
    }
  );
}

[download]
At the time we were doing lots of MS stuff, fortunately those days are now long past, but maybe this will help.

jdtoronto

In reply to Re: Adding MSWord custom properties in Win32::OLE by jdtoronto
in thread Adding MSWord custom properties in Win32::OLE by thoglette

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