use POSIX ':sys_wait_h'; # WNOHANG
my %pids; # Parent's hash of child pids

$SIG{CHLD} = sub {
    local ($?, $!); # Don't change $! or $? outside handler
    until (-1 == (my $pid = waitpid(-1, WNOHANG))) {
        return if $pid == 0; # Processes still running
        next unless delete $pids{$pid};
        say "$pid exited with status $?";
    }
};