Not sufficient. That won't help if the child blocks on printing STDERR, since the parent is blocked reading from the child's STDOUT. 
use warnings;
use strict;
use Symbol qw(gensym);
use IPC::Open3;

my $out = gensym();
my $err = gensym();
my $cmd = 'dir 1>&2';

my $pid = open3(undef, $out, $err, $cmd);
1 while <$out>;
1 while <$err>;
waitpid $pid, 0;

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In reply to Re^2: waitpid on Win32 ... wait forever by ikegami
in thread waitpid on Win32 ... wait forever by syphilis

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