Windows environment variables are case-insensitive. Your 5.003 is buggy, and your 5.8.7 is corrent.

>set myvar=123
>set MYVAR=456
>echo %myvar%
456
>echo %MYVAR%
456
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Update: The bug has been fixed at least since

This is perl, v5.6.1 built for MSWin32-x86-multi-thread
(with 1 registered patch, see perl -V for more detail)

Copyright 1987-2001, Larry Wall

Binary build 633 provided by ActiveState Corp. http://www.ActiveState.
+com
Built 21:33:05 Jun 17 2002
[download]

I can't find any reference to it in the deltas, but it could be mentioned in the Changes file.