For some reason I didn't see that he'd said, "during compilation". Perl is an interpreted language, so there's no compilation stage. A compiled language is modified in this way using a preprocessor. It doesn't really make sense to claim it's possible to inject code during compilation, that's the entire purpose of a compiler, to generate machine code from a syntax tree.

But code generation and execution during runtime, which is what I think you meant, is capable in any language with eval().