in reply to Strange (for me) behavior of Parallel::ForkManager
$manager->finish(\&next); is exit(\&next). Aside from the fact that the argument makes no sense, you're exiting the child at the wrong spot. When that child is done processing a line, it needs to exit by calling finish.
my $manager = new Parallel::ForkManager( 20 );
$manager->start and next;
my (..., $ip, ...) = split( /\s+/, $_ );
print "ip ? $ip\n";
$manager->finish() if !$p->ping($ip);