I was under the impression that you could make perl use the wide system calls by supplying the -C switch. However, perlrun.pod says:

(In Perls earlier than 5.8.1 the -C switch was a Win32-only switch that enabled the use of Unicode-aware "wide system call" Win32 APIs. This feature was practically unused, however, and the command line switch was therefore "recycled".)

I actually had been using that switch in the past. Pity.