Openings in flat slabs near to columns are often needed to supply the building with utilities. The presence of these openings can lead to a decrease of the punching resistance which is related to (i) the reduction of the control perimeter, (ii) the stress concentrations at the edges of the openings, (iii) the reduction of the unitary shear resistance caused by increased flexural deformations, and (iv) the moment transfer in the slab connection in case of unsymmetrical openings. The effect of the openings on the punching shear resistance is strongly related to the geometry, the location, the number, and the size of the openings. To quantify the strength reduction due to the openings, current code approaches account for a reduction of the control perimeter defined by radial lines from the centroid of the column to the edges of the opening. Even though slabs with opening have been investigated since the 1960s, available experimental evidence on the behavior of slabs with openings without shear reinforcement is scarce and some effects are neglected in current design codes. Eurocode 2 and fib Model Code do not consider the effect of unsymmetrical openings leading to moment transfer in the slab-column connection. ACI 318:2019 recommends considering these cases as free edges; however, additional information is necessary to perform the analysis. The present paper analyses a database of 68 flat slab specimens with openings according to ACI 318:2019, current Eurocode 2 (EN 1992-1-1:2004), the draft for the second generation (prEN 1992-1-1:2021), fib MC 2010:2013, and the Critical Shear Crack Theory. This paper proposes a new approach and a new definition of the control perimeter to improve the prediction of the punching shear resistance of slabs with openings based on the results of the database, the previous studies, and the linear-elastic analyses of the shear force distribution along the control perimeter.