Note that you should never print inside the subroutines of signals, thus, for production code:

BAD:

local $SIG{'CHLD'} = sub { print "dead child\n"; $timeout=0 };
[download]

GOOD:

local $SIG{'CHLD'} = sub { $timeout=0 };
[download]

Also, in the above code, $pid == $child_pid (only $child_pid is send from the child to the parent in a IPC)

So if you look at the code from perlplexer in

Perl IPC: Checking Exit Status of Child Process

This is something you actually want:

#!/usr/bin/perl 
use strict;
use warnings;
 
my $timeout = 10;
my $parent_pid = $$;
my $child_pid = fork();

die "Can't fork() : $!\n" unless defined $child_pid;

if ($child_pid){
    # parent
    eval {
        local $SIG{ALRM} = sub { die "timed-out\n" };
        alarm $timeout;
        if (waitpid($child_pid, 0) > 0){
            my ($rc, $sig, $core) = ($? >> 8, $? & 127, $? & 128);
            if ($core){
                print "$child_pid dumped core\n";
            }elsif($sig == 9){
                print "$child_pid was murdered!\n";
            }else{
                print "$child_pid returned $rc";
                print ($sig?" after receiving signal $sig":"\n");
            }
        }else{
          print "$child_pid... um... disappeared...\n";
       }
       alarm 0;
   };
   if($@ eq "timed-out\n"){
        print "$child_pid... parent got bored waiting for it...\n";
   }
   
}else{
    # child
    print "child $$ is running...\n";
    my $ret = system("sleep 20");
    exit $ret;
}
[download]

edit: added alarm

edit2: After reading more about what you actually are trying to do, you also need to loop over the waitpid in such a way that it returns the pid of the child that is ready. Implementing multi queued children is a bit harder. And would require more details about what you require, as, for example, you can prestart 10 children, that are idle until some work for them is done, or you have a single activity queue and a maximum of X children to do the work, and spawn them on the fly when they need to do something.

Checking which child is ready to give you their exitcode can be done with WNOHANG, see waitpid

Depending on what you want to do, independant threads are also an option: threads

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