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$ perl -lwe 'open STDOUT, ">&=6" or die ">&=6: $!"'
>&=6: Bad file descriptor at -e line 1.
$ perl -lwe 'open STDOUT, ">&=6" or die ">&=6: $!"' 6>/dev/null

# Don't use this - DOESN'T WORK
open my $to_child, ">&=6" or die ">&=6: $!";
open my $from_child, "<&=7" or die "<&=7: $!";
...
} else {
    exec { "sub-program" } "sub-program", @args;
}