Comparing compilation of strict.pm and warnings.pm to having to access tied variables is unfair. You are strictly correct that saying use strict; isn't zero cost. The first time that is said anywhere in the program, the strict.pm file has to be loaded and compiled. Then everytime use strict; is said, strict->import; is called once during compilation. Strictly speaking, that's not free. It is, however, as cheap as anything ever gets in perl. It sets two scalar values. That's it. There is no additional runtime overhead. In all terms I've ever come to recognize, strict and warnings are free because any module you load will at some point have loaded it.

Tied variables incur all the same costs but they also cost an additional function call for every time the variable is accessed. They also cost a little bit when it comes to magic resolution and a little bit more if your ISA cache isn't up to date.