Retrieving command exit code with open3

I'm using code similar to the following to run a command and grab the
output from both STDOUT and STDERR separately:

local (*IN, *OUT, *ERR);
my $pid = IPC::Open3::open3(*IN, *OUT, *ERR, $cmd, @args);
print IN join "\n", @input;
close IN;
my @out = <OUT>;
my @err = <ERR>;
close OUT;
close ERR;
waitpid $pid, 0;
print "exit: $?\n"

Most of the time this works, but occasionally waitpid will return -1
and $? will also be set to -1 meaning that there is no child process.
Presumably the child exited before my call to waitpid.

What can I do to reliably grab the exit code?

Thanks,
-Ryan

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