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[
  [[0, 0], [0, 1], [0, 2], [0, 3]],
...
  [[2, 0], [2, 1], [2, 2], [2, 3]],
  [[3, 0], [3, 1], [3, 2], [3, 3]],
]

my $queue = Thread::Queue->new();
$queue->enqueue(\@rows);
my @threads = map { threads->create( \&process, $queue ); } 1 .. 4;
...

if ( $div == 0 ) {
    $image->setpixel( x => $x, y => $y, color => $black );
...
else {
    $image->setpixel( x => $x, y => $y, color => $palette[$color] );
}