Pearled barley must be gelatinized before mashing.
Flaked grains are cooked during the process. The pressures involved in making the flakes creates heat, which cooks each corn individually.
Pearled barley is processed to remove the husk and some of the bran - it's polished, or pearled. You still have to cook it to release the starches which the enzymes in the malted grain can convert.
Essentially, you cook it according to the directions on the package. The stuff I get, I boil gently for one hour, after which the steaming glop is added to the main mash. Don't forget to account for the fluid in your mash calculations.