iRule for pool selecting

You don't need an iRule to do this if the goal is as simple as "use these pool members until none are available, then use these other pool members." You can use Priority Group Activation instead. Define all the nodes in one pool. Make the pool members that are in DC_W a higher priority than the pool members in DC_P. Specify the minimum as not less than the number of members in pool DC_W. If and when the number of members in the higher priority group (DC_W) falls below the minimum, the members in the lower priority group (DC_P) will be activated.

Several articles on Priority Group Activation can be found here on DevCentral, in case you need more information to get started. And it's way more efficient than using an iRule.