my @portlist = qw/10001 10002 10003 10004/;

use Parallel::ForkManager;
$pm = Parallel::ForkManager->new(scalar @portlist);

foreach my $target (@targets) {
    my $pid = $pm->start and next; 

    my $port = pop @portlist;
    system("telnet $target $port");
    push @portlist, $port;

    $pm->finish; # Terminates the child process
}