Add new user and add meta at once

wp_insert_user returns your new user’s ID, if created successful. You can add the user_meta to this ID:

$userid = wp_insert_user( $userdata );
if ( !is_wp_error( $userid ) ) { // check if insert was successful
    add_user_meta( $userid, 'verification_ref', $ref ); // add the meta
} else {
    /* Error Handling */
}