How do I change a script that checks refering URLs from within to a seperate file with the require function?

Current script:

@referers = 'http://www.mydomain.com','http://www.another.com');
# Check Referring URL
&check_url; 
sub check_url {
        local($check_referer) = 0;

    if ($ENV{'HTTP_REFERER'}) {
        foreach $referer (@referers) {

             if ($ENV{'HTTP_REFERER'} =~ /$referer/i) {
                $check_referer = 1;
                last;
            }
        }
    }
    else {
        $check_referer = 1;
    }
    if ($check_referer != 1) { &error('bad_referer') }
}
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Originally posted as a Categorized Question.