1. Which are the most commonly used prefixes for units measuring length?

kilo − as in kilometre. 1 kilometre = 1000 metres 
centi − as in centimetre. 100 centimetres = 1 metre
milli − as in millimetre. 10 millimetres = 1 centimetre

2. Who first decided how long a metre would be?

Originally the French decided a metre would be one ten millionth of the distance from the Equator to the North Pole. It is now given by the wavelength of certain radioactive waves which can be measured very accurately.

3. Are kilometres still used to measure really large distances?

Yes, even though you could use a unit such as a megametre, for say, the circumference of the Earth, it is usually given in km.

4. What units are perimeters measured in?

Because a perimeter is the measure of the distance around an object, it is a length, and its units would be mm, cm, m, km etc.

5. What other units are used for measuring length other than metric units?

In the United States , the system used is called the Imperial system and uses miles, yards, feet and inches.

1 foot = 12 inches
1 yard = 3 feet
1760 yards = 1 mile

This is an old system and a yard was given as the length of a man's arm! It would be very expensive for the US to change to the metric system.