When something happens perceptibly, you can notice it — even if the change is only a little. The room became perceptibly warmer. Her hands were perceptibly shaking. The word is careful: it does not say the change is big, only that it is there and you can tell. In everyday talk people more often say “noticeably.” Though actually, it is also common in the negative: “not perceptibly different” means you can hardly tell the two things apart. You will meet it more in writing than in casual speech.