When you write to $result to make the in-memory png, are you sure it is valid? Can you also write it to a file, and check it?

Also, the docs say that

$png = $pdf->image_png($file)
    Imports and returns a new PNG image object.
[download]

but, $file may mean a disk file. Quite possibly you cannot use an in-memory image. The docs would probably say $data if they accepted binary streams.

You might look into whether you can convert to Base64encoded images, and whether you can use that format. Or use File::Temp and just write to a temporary disk file.