When using truecolor images, I've found that I can skip the colorAllocate() step by specifying the packed rgb value inplace of the index. I use

sub rgb{ unpack 'N', pack 'xCCC', @_ }
[download]

for this, though the name is probably a misnomer.

I'm not sure if this is documented anywhere, and cpan seems to be down at the moment, so I haven't been able to check. It seems to work okay with GD v2.30. It's also a lot faster and more convenient than using colorAllocate() to build an rbg->index table.

This is a simpler, faster version of your original that uses the above:

#!/usr/bin/perl -slw
use strict;
use GD;

sub rgb{ unpack 'N', pack 'xCCC', @_ }

my $width = 800;
my $height = 600;
my $img = new GD::Image( $width, $height, 1 );

for my $x ( 0 .. $width -1 ) {
    for my $y ( 0 .. $height -1 ) {
        $img->setPixel( 
            $x, $y, rgb( $x * 255 / $width, $y * 255 / $height, 0 ) 
        );
    }
}

open my $fh, '>:raw', 'picture.png' or die "uh oh spaghettio : $!";
print $fh $img->png;
close $fh;

## display result on win32 systems
system 'picture.png';
[download]