If your process gets two or more SIGCHLD signals before being scheduled, only one signal will arrive. This because the kernel keeps track of the signals to your process in a bit vector, one bit per uniq signal. The solution is to loop, like this:

sub REAPER {
    my $pid;
    while(( $pid = waitpid(-1, WNOHANG)) > 0) {
        # Do stuff
    }
    $SIG{CHLD} = \&REAPER;
}
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