actually, perceived exertion is cheaper , but you are right on with the HRM.

OP, it is true that you'll likely overdo an interval at its beginning or climb at a power way above your threshold power on even a 3% hill if you use only an HRM, but these are the things you learn to deal with and adapt to after a while. Case in point: say you are doing a zone 2 ride (for which efforts should stay below threshold), and the top of your endurance zone is 145. You don't gun it so that it ramps to 145 immediately because you are likely putting out power in your VO2 max zone when you do it. Instead, your goal is to hit the bottom at 141 and slowly let your HR creep up. The effect of this would be very similar to metering your efforts so your power output is in high tempo.

When you do enough intervals with a HRM you learn how the ramping of your HR should be so that you won't be overdoing anything. As for the short, 1minute intervals, well, those should be all out anyway that if you are looking at your PM, you are doing something wrong.