I'm a bit late but perhaps this might be still of interest:

Using splice for building the subqueues:

my @mp3s = ( 'a' .. 'z' );

my $cores = 4;

my @queue;

push @queue, [ splice @mp3s, 0, $cores ] while @mp3s;
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Or an iterator with List::MoreUtils:

use List::MoreUtils qw (natatime);

my @mp3s = ( 'a' .. 'z' );

my $cores = 4;

my @queue;

my $iterator = natatime $cores, @mp3s;

while ( my @buff = $iterator->() ) {
    push @queue, \@buff;
}
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This yields:

$VAR1 = [
          [
            'a',
            'b',
            'c',
            'd'
          ],
          [
            'e',
            'f',
            'g',
            'h'
          ],
          [
            'i',
            'j',
            'k',
            'l'
          ],
          [
            'm',
            'n',
            'o',
            'p'
          ],
          [
            'q',
            'r',
            's',
            't'
          ],
          [
            'u',
            'v',
            'w',
            'x'
          ],
          [
            'y',
            'z'
          ]
        ];
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Then:

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
    }
}
[download]

Regards, Karl