in reply to No child processes

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.

You may, therefore, want to set the signal handler to do something useful like:
my %pid; $SIG{CHLD} = sub { while((my $kid = waitpid(-1,WNOHANG))>0 ) { warn "PID $kid returned $?"; } };

(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 ...