use Parallel::ForkManager;

my $cores = 4;

my $pm = Parallel::ForkManager->new($cores);

foreach my $child (@queue) {
    $pm->start and next;
    process($child);
    $pm->finish;
}

$pm->wait_all_children;

sub process {
    my $mp3s = shift;
    foreach my $mp3 (@$mp3s) {
       # do the stuff
    }
}