I just nick the edge in line with a row of holes, then bend it starting from the nick - it snaps along the selected row of "perforations".
For better accuracy (or with fibreglass laminate stripboard), score both sides heavily along a row of holes with a sharp knife & steel rule as a guide, then snap along the score.
File the cut edge to smooth it & remove any ragged bits.
I find that with some type of board, trying to use actual cutters (eg/ tinsnips or similar) causes cracking away from the cut line, due to the sharp bending at the cutting point.