Thank you for posting this. I was able to speed it up some by storing the RGB pixel values in 2D arrays. The image that I am testing originally took 28 seconds to process and I have it down to 12 seconds. I'll cut-n-paste the parts that I changed:

    my $gray      = 0;
    my ( $width, $height ) = $imSrcImg->Get( 'width', 'height' );

    my @pixels_red;
    my @pixels_green;
    my @pixels_blue;
    for (my $x = 0; $x < $width; $x++) {
       for (my $y = 0; $y < $height; $y++) {
          (my $red, my $green, my $blue) = split(',', $imSrcImg->Get("
+pixel[$x,$y]"));
          $pixels_red[$x][$y] = $red;
          $pixels_green[$x][$y] = $green;
          $pixels_blue[$x][$y] = $blue;
       }
    }

    # Iterate over every pixel in the image and change
    for my $y ( 1 .. ( $height - 2 ) ) {
        for my $x ( 1 .. ( $width - 2 ) ) {
            for ( my $i = -1 ; $i <= 1 ; $i++ ) {
                for ( my $j = -1 ; $j <= 1 ; $j++ ) {
                    my $r = $pixels_red[$x + $i][$y + $j];
                    my $g = $pixels_green[$x + $i][$y + $j];
                    my $b = $pixels_blue[$x + $i][$y + $j];

                    my $tmp_GX = $GX[ $i + 1 ][ $j + 1 ];
                    my $tmp_GY = $GY[ $i + 1 ][ $j + 1 ];

                    if ($tmp_GX) {
                       $sumRx += $r * $tmp_GX;
                       $sumGx += $g * $tmp_GX;
                       $sumBx += $b * $tmp_GX;
                    }

                    if ($tmp_GY) {
                       $sumRy += $r * $tmp_GY;
                       $sumGy += $g * $tmp_GY;
                       $sumBy += $b * $tmp_GY;
                    }
                }
            }
[download]

The rest of the code is the same.

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