I'm even curious to know why a flag like _PROTECT_FROM_CLOSE was created in the first place. :)
Sometimes two processes need to communicate via file descriptors and std* is not enough- Unix example I know of is UCSPI (ooks-pie) UNIX Client-Server Program Interface
which qmail uses. In a case like that, you need to keep non-STD* filehandles from closing when you exec a new process, otherwise the child tries using FD 3 (or FDs 6 and 7, etc) and finds it is closed.
Unix has no problem with this, it uses fcntl($fh, F_SETFD, ~FD_CLOEXEC) and all is well. But those POSIX macros are not implemented in Win32-land, at least not in ActiveState's Perl. Hence my desire to try _PROTECT_FROM_CLOSE on FD 3.
I was all over the web looking for insight, tried other tricks like setting $^F=3 (never even glanced at that perlvar before) but no luck. The best I found was a terse comment that said "Can't be done. OS limitation." which gave me the courage to give up!
Thanks for looking into it, and confirming that I'm not alone.
Here is my original goal, which seems to be elusive to many on the web: have a program write to stdout, stderr, and some 3rd stream. In Unix it would be invoked like "MyProg >stdout.log 2>stderr.log 3>extra_data.log". Windows cmd.exe shell allows FD's 3-9 in redirects but it doesn't pass them to any programs it spawns, they seem to be just for dup'ing 0,1,2 on the command line.
My idea was to write a wrapper in perl that would allow communicating with a sub-program using those higher-number, user-defined file descriptors in Win32.