Hmmm... well... I just tested and next if $response == 500 works as expected:

use strict;
my @responses = (500, 300, 200, 400, 500, 300, 200, 500, 400, 500);
while (<DATA>) {
    chomp;
    my $response = $responses[$_];
    unless ($response == 200) {
        next if $response == 500;
        print "[$_] $response\n";
    }
}

__DATA__
0
1
2
3
4
5
6
7
8
9
[download]

outputs, as expected:

[1] 300
[3] 400
[5] 300
[8] 400
[download]

correctly skipping the responses which are either 200 or 500... so your problem might actually be another part of the code...

acid06
perl -e "print pack('h*', 16369646), scalar reverse $="