You can use the good ole hacksaw, but there is a better way.
The hacksaw leaves burred edges and can be unwieldy, but a good set of PVC cutters will slice thru PVC and black poly easily and give a clean cut every time.
They last for ages and you can pick up a pair at any retic shop. Most cutters suit up to 25ml pipe, but you can get some that will do up to 40ml pipe also.
How to Cut PVC Pipe (Easily)