i guess that under windows fork is implemented using threads so the child does not have his own STDOUT.

Why are you dont you open a new filhandle in each child process

Update: Gee, Koosemose appears to be right, this does seem to work right (at least on Win98):

open CHILDOUT, ">test.txt";
select CHILDOUT;
system('perl -le "print q(Hello!)"');
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I think that would indeed solve it.