Works for me. Are you sure your child is exiting with something other than zero?

$ cat a.pl
use strict;
use warnings;

use POSIX qw( WNOHANG );

my $pid = fork();

if ($pid == 0) {
   sleep(5);
   exit $ARGV[0];
}

while (waitpid($pid, WNOHANG) == 0) {
   sleep 1;
   print time(), "\n";
}

print "$?\n";
[download]

$ perl a.pl 123
1288161738
1288161739
1288161740
1288161741
1288161742
31488
[download]

(31488 == 123 * 256)