my @waiting = @chromosomes;
my %threads;
my $max_threads = 10;

## Continue to join threads as the become joinable 
## whilst there are still threads running
while( @waiting or threads->list( threads::running ) ) {
    if( keys %threads  < $max_threads ) {
        my $chr = pop @waiting;
        $threads{ $chr } = threads->new( \&threading_function, $chr );
    }
    ## Do this regardless of whether it's time to start a new thread.
    foreach my $chr ( keys %threads ) {
        if( $threads{ $chr }->is_joinable ) {
            $threads{ $chr }->join;
            delete $threads{ $chr };
        }
    }
    sleep 1; ## Prevent this thread consuming all spare cpu
}