All I can provide is a negative data point. Using your code on a random MS Project file, also under Win XP, didn't produce any error. Here's a transcript (I added print "Made it" to the end of your snippet):

Microsoft Windows XP [Version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.

H:\>perl washizu.pl
Opening H:\test.mpp
Made it

H:\>perl -v

This is perl, v5.8.4 built for MSWin32-x86-multi-thread
(with 3 registered patches, see perl -V for more detail)

Copyright 1987-2004, Larry Wall

Binary build 810 provided by ActiveState Corp. http://www.ActiveState.
+com
ActiveState is a division of Sophos.
Built Jun  1 2004 11:52:21
 ...
[download]

I also have MS Project 2000 installed (9.0.2001.0219 SR-1 according to the "Help About").

Good luck. If you figure it out, update your node or post a reply.