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use constant CODE=>0;
use constant SUBCODE=>1;
...

use constant {
    CODE=>0, SUBCODE=>1, ....
};

if( $response[CODE] == 20 and 
     $response[SUBCODE] == 55 ) {
     # Do something appropriate
}

if( "@response[CODE, SUBCODE]" eq '20 55' ) {
    # do stuff
}