This is the next tutorial in my Learning XS series. This post explores one of Perl's most powerful optimisation techniques: Custom Ops . We'll build a real-world example, reimplementing one of my first XS cpan modules - a Shannon entropy calculator - and explain how to bypass Perl's subroutine call overhead entirely. So firstly... What are Custom Ops? When you call a Perl subroutine, Perl executes an entersub op that: Sets up a new stack frame Pushes arguments onto the stack Magical things happens Jumps to the subroutine code More Magical things can happens Cleans up and returns For simple functions called millions of times (like mathematical operations), this overhead dominates execution time. Custom ops let you replace the entire entersub call with a single, specialised op - eliminating that overhead. Next.. What is Shannon Entropy?…