I am also going to Peru on 27th August (via Madrid) and I am still not absolutely sure what mix of clothes to take. If you haven't already done a search this may help.
I looked up temperatures in the three key locations for Aug/Sept (Day High, Day Low, Wet Days), results as follows. I hope it makes sense:
Lima (17°, 13°, 1 or 2)
Cuzco (20°, 1°, 2 to 6)
Iquitos (30-35°, 13°)
I suppose these to be average temperatures over 24 hours, but from reading the text I would guess that day low (particularly Cuzco) actually means after sunset, i.e. night time, but it's not exactly clear on any of the web sites I looked at or even in the guide book.
Also the recommended Clothing/Essential list in the Justyou holiday documents seems to match quite closely with suggested items I have found in other places.
Hope this all helps.
Barry