use POSIX qw( _exit );

my $num_drives = 3;
my $num_discs = 12;

my @idle;  # Idle drives
my %busy;  # Busy drives, keyed by the pid using them

sub wait_for_a_kid {
    my $pid = wait();
    if ($pid == -1) {
        warn("Can't wait: $!\n");
        return 0;
    }
    ... check the child's exit code if desired ...
    push @idle, delete($busy{$pid});
    return 1;
}

for my $drive (1..$num_drives) {
    ... eject disc ...
    push @idle, $drive;
}

my @queue = 1..$num_discs;
while (@queue) {
    if (!@idle) {
        last if !wait_for_a_kid();
    }

    my $disc = shift(@queue);
    my $drive = shift(@idle);
    my $pid = fork();

    if (!defined($pid)) {
        warn("Can't fork: $!\n");
        last;
    }

    if ($pid) {
        $busy{$pid} = $drive;
        next;
    }

    if (!eval {
        print("Please insert disc $disc in drive $drive\n");
        ... wait for disc to be present ...
        ... read disc ...
        ... eject disc ...
        1;
    }) {
        warn($@);
        _exit(1);
    }
    _exit(0);
}

while (keys(%busy)) {
    last if !wait_for_a_kid();
}
[download]

Update: Added some error checking. Fixed a bug exit wait loop.