In the context of increasing the performance of structured packing used in distillation columns, we develop a state-of-the-art flow diagnostic method to investigate the intricate fluid dynamics of a falling film interacting with a perforation. Perforations are one of the several key microstructure elements in industrial structured packing whose role in mass transfer enhancement and liquid redistribution is yet not well understood. We focus the present investigation on the liquid curtain, or "twin film", blanketing the perforation. Twin films are prone to strong capillary waves and recirculation zones, and experiments of fundamental nature are required to assess the fluid dynamics in the curtain formation in the perforations and ultimately uncover mechanisms that intensify interfacial mass transfer. Here, a three-dimensional two-component (3D-2C PTV) optical-flow-based technique using the concept of defocus is implemented on a test bench designed to study the effect of perforations on falling liquid films. This method provides an opportunity to evaluate the hydrodynamics of the liquid curtain through streamline patterns and dynamic modes and to describe key features that could promote mass transfer intensification in structured packing.