It won't work as written. Neither coders nor decoders are stable.

Does your module (can't find it on CPAN) inherit from GD (judging by width/height/jpeg methods)? It doesn't matter in the end, I only hope you force it to treat jpegs as truecolor on open, because it doesn't despite whatever its doco says. If GD converts jpeg to palette, it will be even more mess to add to description below -- looks like GD tunes this algo (true to palette quantization) more frequently, then no need to go as far back as 5.010 to demonstrate.

Frog is frog

use strict;
use warnings;
use feature 'say';
use GD;
use Digest::MD5 'md5_hex';

say $^V; 
say $GD::VERSION;

my $f = 'frog.jpg';

GD::Image-> trueColor( 1 );
my $i = GD::Image-> new( $f );
say "Image is ", ( $i-> isTrueColor ? '' : 'not ' ), 'truecolor';

printf "RGB triplet for 0,0 pixel is: %3\$d, %2\$d, %1\$d\n", 
    unpack 'C3', pack 'L', $i-> getPixel( 0, 0 );

__END__

v5.32.1
2.76
Image is truecolor
RGB triplet for 0,0 pixel is: 0, 248, 231

v5.10.1
2.44
Image is truecolor
RGB triplet for 0,0 pixel is: 0, 247, 231
[download]

Blame ancient GD version? But

>convert frog.jpg -format "%[pixel:u.p{0,0}]\n" info:
srgb(0,247,231)
[download]

I'd say lossy codecs are murky waters and avoid them in tests:

>convert frog.jpg frog.png
[download]

use strict;
use warnings;
use feature 'say';
use GD;
use Digest::MD5 'md5_hex';

say $^V; 
say $GD::VERSION;

my $f = 'frog.png';

GD::Image-> trueColor( 1 );
my $i = GD::Image-> new( $f );
say "Image is ", ( $i-> isTrueColor ? '' : 'not ' ), 'truecolor';

say md5_hex( $i-> png() );

__END__

v5.32.1
2.76
Image is truecolor
1b6edecaa6d0b67f7bf960113f2136c7

v5.16.3
2.49
Image is truecolor
9edf3f9e11991f2dcfd77a7e1ffbafe8
[download]

Eh? What's the matter now? My first thought was zlib tunes its compression algo (despite same "level" 0..10), but it's not the reason for difference above -- though I strongly suspect it can influence the result for some input other than puny frog. But, above, it's just pHYs chunk that libpng decides to include from some version on.

Then, what it all amounts to -- try stable (read: "obsolete") lossless coder with uncompressed output (GD can't dump raw pixels, unfortunately), short of enumerating pixels one by one and adding RGB to string. Hopefully it's OK now:

use strict;
use warnings;
use feature 'say';
use GD;
use Digest::MD5 'md5_hex';

say $^V; 
say $GD::VERSION;

my $f = 'frog.png';

GD::Image-> trueColor( 1 );
my $i = GD::Image-> new( $f );
say "Image is ", ( $i-> isTrueColor ? '' : 'not ' ), 'truecolor';

say md5_hex( $i-> gd() );

__END__

v5.32.1
2.76
Image is truecolor
e60c6afd7eefe80050d6af4488457281

v5.16.3
2.49
Image is truecolor
e60c6afd7eefe80050d6af4488457281

v5.10.1
2.44
Image is truecolor
e60c6afd7eefe80050d6af4488457281
[download]