use threads;

use Thread::Queue qw( );
         
my $num_drives = 3;
my $num_discs = 12;

my $q = Thread::Queue->new();
$q->enqueue( 1..$num_discs );
$q->enqueue( (undef) x $num_drives );

for my $drive (1..$num_drives) {
    ... eject disc ...
    async {
        while (defined(my $disc = $q->dequeue())) {
            print("Please insert disc $disc in drive $drive\n");
            ... wait for disc to be present ...
            ... read disc ...
            ... eject disc ...
        }
    };
}

$_->join for threads->list();