Change &WNOHANG to WNOHANG() or WNOHANG and then yell at whoever taught you to use &NAME for constants in hopes that this problematic but still common practice can be stomped out.

FYI, the reason you get this warning is that your sub reaper gets called with the name of the signal passed in to it and calling a subroutine (and constants from Perl modules are usually subroutines) with & and without parens means that the current value of @_ is reused as the arguments to that subroutine.

So you end up doing the equivalent of WNOHANG(@_) which, in your case, is similar to WNOHANG("CHLD"). Finally, for obscure reasons, the example code for implementing constants in XS modules, allows these so-called "constants" to take an optional numeric argument. But "CHLD" isn't numeric, hence the warning.

Unfortunately, this culprit looks like the fine manual. The example in perlfunc (5.6.0) reads:

use POSIX ":sys_wait_h";
#...
do {
    $kid = waitpid(-1,&WNOHANG);
} until $kid == -1;
[download]

Worse yet, the docs for POSIX in 5.7.2 give this example:

$pid = POSIX::waitpid( -1, &POSIX::WNOHANG );
print "status = ", ($? / 256), "\n";
[download]

You want to submit a doc patch to p5p or should I? :)