I've worked quite a bit with Outlook but not the express one. Have you had a look at the object model for further reference ? I know you can test to see which sofware you're using and depending on the software choose the appropriate code.
It seems to me you're not using OLE at all. It would probably make it all easier if you had a look at Win32::OLE and Outlook's object model (that is if Express uses the same). Once you access OLE properties, it's really easy to access the reply address. Try this piece of code :

   if ( my $outlook = Win32::OLE->GetActiveObject('Outlook.Application
+'))# connect to outlook application
   {
      if (defined ($outlook->ActiveInspector()))
      {
         my $activeEmail= $outlook->ActiveInspector()->{CurrentItem};
         if ($activeEmail->{Class}==olMail) # if we're dealing with an
+ email item
         {
            my $emailProperties;
            $emailProperties->{Recipients}    =$activeEmail->{Recipien
+ts};
            $emailProperties->{ReplyRecipients}    =$activeEmail->{Rep
+lyRecipients}->ResolveAll();
         }
      }
   }
[download]

Good luck...