Reap child in case where exception has been thrown

If open3 throws due to an issue such as an exec failure, the caller
cannot know the child PID to wait for. Therefore it is our
responsibility to reap it.

Also update POD, since on some platforms exec failures now ARE raised as
exceptions (since perlbug #72016).

ext/IPC-Open3/lib/IPC/Open3.pm

@@ -57,7 +57,8 @@ as file descriptors.
 open3() returns the process ID of the child process.  It doesn't return on
 failure: it just raises an exception matching C</^open3:/>.  However,
 C<exec> failures in the child (such as no such file or permission denied),
-are just reported to CHLD_ERR, as it is not possible to trap them.
+are just reported to CHLD_ERR under Windows and OS/2, as it is not possible
+to trap them.
 
 If the child process dies for any reason, the next write to CHLD_IN is
 likely to generate a SIGPIPE in the parent, which is fatal by default.
@@ -297,6 +298,7 @@ sub _open3 {
 	    if ($bytes_read) {
 		(my $bang, $to_read) = unpack('II', $buf);
 		read($stat_r, my $err = '', $to_read);
+		waitpid $kidpid, 0; # Reap child which should have exited
 		if ($err) {
 		    utf8::decode $err if $] >= 5.008;
 		} else {

ext/IPC-Open3/t/IPC-Open3.t

@@ -14,10 +14,11 @@ BEGIN {
 }
 
 use strict;
-use Test::More tests => 37;
+use Test::More tests => 38;
 
 use IO::Handle;
 use IPC::Open3;
+use POSIX ":sys_wait_h";
 
 my $perl = $^X;
 
@@ -154,6 +155,10 @@ $TB->current_test($test);
     isnt($@, '',
 	 'open3 of a non existent program fails with an exception in the parent')
 	or do {waitpid $pid, 0};
+    SKIP: {
+	skip 'open3 returned, our responsibility to reap', 1 unless $@;
+	is(waitpid(-1, WNOHANG), -1, 'failed exec child is reaped');
+    }
 }
 
 $pid = eval { open3 'WRITE', '', 'ERROR', '/non/existent/program'; };