I doubt that's your problem, but here is what I'd use, with a diagnostic: 
next if $response == 500;
warn "->$response<-\n";

[download]
For what it's worth, tye had the best suggestion in that thread. You might also change your loop condition slightly: 
while (defined( my $url = <IN> )) {
    chomp $url;

[download]

In reply to Re: using unless($code == 200).... by chromatic
in thread using unless($code == 200).... by RayRay459

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