Why not just repeat what you did to get the HTML? Probably something along these lines:

$data = get $UriImage;

open(FD, ">wakawall");
binmode FD;
print(FD, $data);  
close FD;

copy ("wakawall", "~/wakawall.jpg");

Don't forget to include File::Copy, nor your die()s (nor strict mode...).

I've forgotten most of perl though, so I defer to anyone who actually knows how to beat it into submission.