This paper presents a detailed analysis examining the absorption performance of a metal-dielectric slab with subwavelength size periodic perforations exploiting quarter-wave impedance matching (QWIM) technique within long wave infrared (LWIR) regime (8-12µm). Integration of perforations to a simple stack with various period sizes and perforated area ratios are examined through theory, simulation, and measurements that are in great agreement. Advantages of perforated absorbers for thermal detectors are discussed in maximizing optical absorption and reducing thermal-mass point of view. Introducing perforation in umbrella type absorbers is mainly employed for reducing the thermal-mass while maintaining the high absorption performance. Within the scope, it is experimentally shown that a perforation ratio (width/period) of 50% with square holes for the umbrella layer is possible without degrading the maximum LWIR absorption performance of 96% when the sheet resistance of Rs=400Ω/□ is employed for the absorbing metal layer, which is close to free space impedance of 377Ω/□. Nevertheless, this ratio can be increased up to 77% by depositing a thicker absorber metal with smaller sheet resistance, such as Rs=100Ω/□ while still maintaining an average absorption performance of 93%, which are all predicted numerically by simulations and physically explained through effective medium approach (EMA).