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Accessing behind casing opportunities plays a key role in ensuring that production targets are achieved in the PETRONAS operated fields in Malaysian basin. This is however getting more challenging as these fields comprising mostly of stacked clastic reservoirs get matured. This paper provides a comprehensive review of all thru tubing perforation jobs done over last two years in PETRONAS operated fields in the region with focus on failure reasons and future mitigation measures to ensure higher rate of success. As a part of this study, all the historical thru tubing perforation jobs done to access behind casing opportunities over last 2 years across more than 22 producing fields in the region with PETRONAS as the operator were analyzed in detail. These jobs were then studied with respect to the main factors which impacts overall job success or failure. These included but was not limited to fluid contacts, gun, and charge type, nearby well performance, zone saturation data availability, completion type, contractor performance etc. In addition, post job details like sand production, artificial lift performance which contributed to well behavior post perforation were also evaluated. Exhaustive gun perforation and well model simulations were conducted to model post perforation well performance. Actual data with distribution and impact of the above-mentioned parameters will be discussed at length in the paper. Key findings which contribute to the overall successful thru tubing perforation jobs and lessons learnt for future perforation jobs will be presented in the paper. Based on the comprehensive review and in-depth analysis, the operator has gained great visibility on key focus areas for thru tubing perforation jobs to increase success rate for completing the behind casing opportunities. Results specially re-iterate the importance of saturation data, gun selection and sand control among other parameters for successful thru tubing perforation jobs in matured stacked reservoirs. The study also indicate why these parameters are even more important for multiple tubular perforations such as cement packers, complex dual string completions, multiple tubular perforation etc. Based on the findings from the study, the paper will recommend key points to consider while completing behind casing opportunities in challenging environment. While a lot of literature and study is available on perforations in general, the author could not find any noticeable work which is based on large amount of actual field data on how to improve thru tubing perforation jobs in matured stacked reservoirs in complex completions. This paper aims to help operators improve on the existing practices to ensure better and more successful results while completing behind casing opportunities using thru tubing perforations.