Flexible rockfall protection barriers are used ubiquitously to safeguard people and infrastructure against falling rock fragments along weak or fractured slopes. Performance of these barriers is often quantified in terms of the level of impact (kinetic) energy that can be withstood, referred to here as the "critical energy." As pointed out in recent papers, however, there is no single representative value of critical energy for a given barrier. Instead, critical energy decreases as the block size decreases, a phenomenon referred to as the "bullet effect." With a view towards explaining and predicting the bullet effect, the paper presents a simple analytical model for perforation of a flexible barrier caused by normal impact. The model rests on a two-dimensional idealization of the full three-dimensional impact problem, as well as a balance of energy between the initial kinetic energy of the block and the energy absorbed by the barrier. The model predicts a strong dependence of the critical energy on the block size, and the predicted trend agrees well with data available in the literature. Ramifications for practical applications and possible future refinements to the model are also discussed.