The way the waivers work, you are not penalized if your guy is gone before it is your turn. Put Perez as your top waiver pick, and you can even put a backup choice SP as your second pick. If Perez is gone by your first-round pick, you wll get your second pick instead. Of course if you do get Perez with your first pick, the system will still try to give you your second pick in the second round.
The order that you have your Waiver Claims in is important. When it's your turn in the Waiver Priority queue, your highest-ranked Claim (of a player that's still available) will be processed.
I managed to trade Maddux for Perez before waivers was over. Was (falsely) accused of cheating. Was it worth it? No...Perez is sucking right now. And so is Clemens!