LINE:
while (<IN>) {
    my $url = $_;
    my $response = 0;

    next LINE if $response == 500;
    unless ($response == 200) {
        # Do stuff here.
    }
}
[download]

Of course, the check for equality to 500 will never work because you set response to 0 and never change it. This means that your unless-clause will always enter the block.

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We are the carpenters and bricklayers of the Information Age.

Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.