Given an IP address I need to determine if it is part of a subnet for whitelist/blacklist purposes.

$allowed = ip_in_subnet('65.182.224.40','65.182.224.0/24');

// Borrowed from http://php.net/manual/en/function.ip2long.php#82397
function ip_in_subnet($ip, $cidr) {
    list ($net, $mask) = explode ('/', $cidr);
    return (ip2long ($ip) & ~((1 << (32 - $mask)) - 1)) == ip2long($net);
}

I also ported this function to Perl:

sub ip_in_subnet {
    my ($ip, $cidr)  = @_;
    my ($net, $mask) = split('/', $cidr);

    my $ret = (ip2long ($ip) & ~((1 << (32 - $mask)) - 1)) == ip2long($net);

    return int($ret);
}

Note: You will need the ip2long function.