When saving msword document after adjusting the custom properties you MUST set the Saved flag : see http://support.microsoft.com/kb/195425/en-us

without it it won't be saved.......

use strict;
use Win32::OLE qw(in);
use Win32::OLE::Const 'Microsoft Office';
#my ( $doc ) = Win32::OLE->GetObject('D:/4nt8/perl-msword_dap/excel_da
+p/Inrichting Arag-ftp.doc');
my ( $doc ) = Win32::OLE->GetObject('//vldfps1/algemeen/FM&O/Klanten/S
+-t/TESTING/Dap Dossier/01. Dap/FBL.OPR.UX.138 WI Inloggen, shutdown e
+n reboot van  LPAR AIX systemen m.b.v. de HMC en PTY.doc');

my $property = 'Arno';
my $value = 'KranenDONK';

my $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;
$doc->{Saved} = 0;   # SEE : http://support.microsoft.com/kb/195425 en
+ http://compgroups.net/comp.lang.perl.misc/how-read-ms-word-summary-p
+roperties/477092
$doc->Save;
$doc-> Close;
[download]

Hope somebody will find this quicker then I did....