iRule to set country code for IP

I'm pretty sure that the whereis function is read-only, so you can't do any kind of setting the value. Based on your original question of wanting to test, then your best bet may be to just use a variable and set it manually

when HTTP_REQUEST {
    if {[IP::client_addr] == "Your.IP.Address"} {
        set country "CN"
    } else {
        set country [whereis [IP::client_addr] country]
    }

    For verification, tail /var/log/ltm as you implement the changes
    log local0. "IP: [IP::client_addr]; Country Code: $country"

     Geo-location iRule continues...
       Use the "$country" variable instead of "whereis" from now on.        
}