Chapter 8. Know Thy Code

Hacks 70-83

Introspection isn’t just a self-help exercise. It’s a way of asking Perl what it thinks about your program.

Why does that matter? There are plenty of advanced techniques that, properly applied, will save you much time, effort, and trouble. That word “properly” is the sticky one though—unless you know what’s proper and what’s not, you’ll have difficulty mastering advanced Perl.

Despite all the rich nooks and crannies and hidden corners of the core, there are only a few techniques you absolutely must understand. Study well the hacks here and you’ll absorb higher lore and unlock secrets that will help you customize Perl, the language, for your specific needs.

Understand What Happens When

Tell compile time from runtime.

Dynamic languages are flexible, neither requiring you to know all of the code you’re ever going to run in a program at compile time nor necessarily failing if it’s not there at runtime. Perl can live with some ambiguity about seeing functions you haven’t defined yet (if ever) and referring to variables that don’t necessarily have any values yet.

That doesn’t always make life easier for programmers. While Perl’s pretty good about knowing what happens when, reading the source code doesn’t always make it clear. While it may seem obvious to you that program execution happens top to bottom that’s not always how it works.

The Hack

Here’s what actually happens.


When you first run your program, Perl reads the file and starts compiling ...

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