You'll want to modify your check_url function so that it accepts a URL or a list of URLs as an argument; then you're not depending on a global variable.

You may also want to take the error-handling and error-invocation out of the subroutine (since it sounds like you're moving the subroutine into a library of some sort) and let your application code deal with the error.

Personally, I'd change the subroutine to accept a list of valid referrers and a URL to check, then return either 1 for a good url or 0 (or undef) for a bad URL.

Something like this:

    sub check_url {
        local $_ = shift;
        my $valid_referrers = shift;

        my $valid = join '|', @$valid_referrers;
        return /$valid/ ? 1 : 0;
    }
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You can use this something like this:

    my @referrers =
       ('http://www.foo.com/',
        'http://www.bar.com/');

    my @urls_to_check = 
       ('http://www.baz.com/bar/',
        'http://www.foo.com/bar/');

    for my $url (@urls_to_check) {
        print $url, ": ", check_url($url, \@referrers), "\n";
    }
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