#!/usr/bin/perl
use 5.010;
use strict;
use warnings;
use POSIX ":sys_wait_h";

$|=1; # autoflush

my %pids;

sub reaper
{
	$SIG{CHLD}=\&reaper;
	# This does not work, because you may get only one signal when
	# two or more processes exit at nearly the same time. This is due
	# to the way signals are implemented.
	#my $pid=wait();
	#delete $pids{$pid};
	my $pid;
	do {
		$pid=waitpid(-1, WNOHANG);
		delete $pids{$pid};
	} while $pid>0;
}

$SIG{CHLD}=\&reaper;
for (1..10) {
	my $pid=fork() // die "Can't fork: $!";
	if ($pid) {
		# parent, remember PID
		$pids{$pid}++;
		say "Forked child $_, PID=$pid";
	} else {
		# child
		$SIG{CHLD}='DEFAULT'; # maybe not needed, but doesn't hurt
		say "Child process $_";
		sleep (5+rand(5));
		exit 0;
	}
}
while (keys %pids) {
	say "Parent is working, ",scalar keys %pids," children still alive (PIDS ",join(', ',keys %pids),")...";
	sleep 1;
}
say "all childs exited";