Do look at this primer on PL/SQL inlining. It tells how it works, and guides its use in practical programs.
It was written by my colleague Charles Wetherell. Charles is the driving force behind the project that brought the brand-new PL/SQL optimizing compiler way back in Oracle Database version 10.1. This was a daring exercise in organ replacement. The operation was an unqualified success, and the patient continues to thrive. With modern optimizing technology in place, the essential foundations are laid for other improvements. Inlining is just such an example.
We've seen various questions about the knobs that fine-tune this feature. Charles's primer answers these within the context of a crystal clear account of how it all works. The main message is that Inlining is a safe and effective optimization that almost always improves application performance and has no important disadvantages. The basic recommendation, therefore, is that all units should be compiled at optimization level 3 – that is, with inlining on -- and that the fine-tuning knobs should be used only sparingly. You must read it. I promise that you'll enjoy doing so.