Instead of enqueueing pixels, you enqueue an offset, and a length of 32 (and the thread will also need the width of the image).
With that data, each thread is able to calculate itself its tasked pixels.

Thus, if I have offset=0 the formula give calculates (x=0,y=0) to (x=8,y=0).
If I have offset=32 the formula give calculates (x=8,y=0) to (x=16,y=0).

Those are 8 pixels of 4 bytes each (RGBA).

you can avoid that thread-save problem with imagemagick.
You read an image and $blob=$original->ImageToBlob(); then manipulate all pixels in the blob, and BlobToImage() back.
See this imagemagick post.
And also this Permonk post: Re: reading in raw data into perl's ImageMagick

At least these primitives are ok with threads? (although not your eyes, you see I used seizure inducing red and green lines... sorry for that)

#! perl
use warnings;
use strict;
use threads;
use Image::Magick;

my $image = Image::Magick->new(
  size => "600x600",
);

$image->Read("xc:white");
for my $i (200..400){
  async{
    my $color = $i % 2 ? '#f00' : '#0f0';
    $image->Draw(
      primitive => 'line',
      points    => "$i,100 $i,500",
      stroke    => $color,
    );
  }->join;
}

$image->Set(magick=>'gif');                                           

my $blob = $image->ImageToBlob();

open(FH,"> $0.gif")or die "$!\n";
print FH $blob;
close FH;
[download]

perldoc threads

Blue rectangle over striped pattern (writing to same pixels), threaded. The Rectangle does not survive intact...

#! perl
use warnings;
use strict;
use threads;
use Image::Magick;

my $image = Image::Magick->new(
  size => "600x600",
);

$image->Read("xc:white");
for my $i (200..400){
  async{
    my $color = $i % 2 ? '#f00' : '#0f0';
    $image->Draw(
      primitive => 'line',
      points    => "$i,100 $i,500",
      stroke    => $color,
    );
  }->join;
  async{
    my $y = 100 + $i;
    $image->Draw(
      primitive => 'line',
      points    => "200,$y 400,$y",
      stroke    => '#00f'
    );
  }->join;
}

$image->Set(magick=>'gif');                                           

my $blob = $image->ImageToBlob();

open(FH,"> $0.gif")or die "$!\n";
print FH $blob;
close FH;
[download]

The example in the previous post with and without threading:

               Rate     threaded non-threaded
threaded     1.41/s           --         -94%
non-threaded 24.7/s        1652%           --
[download]