If you are really determined that you don't want to un-set your CHLD signal handler, you need to bear in mind two things:

i) your 'standard' handler is throwing away the child pid and return code values and
ii) system() is, in effect, shorthand for the traditional UNIX fork/exec/wait idiom.

my %pid;
$SIG{CHLD} = sub {
    while((my $kid = waitpid(-1,WNOHANG))>0 ) {
        warn "PID $kid returned $?";
    }
};
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(this is based Lincoln Stein's 'Network Programming with Perl' p305).
.....

and replace $res = system @args;
with exec @args;

Though, you will need to consider the usual caveats regarding signal handlers in perls prior to 5.8.0 ...