Programming Perl, 3rd Edition

Control Structures

So far, except for our one large example, all of our examples have been completely linear; we executed each command in order. We've seen a few examples of using the short-circuit operators to cause a single command to be (or not to be) executed. While you can write some very useful linear programs (a lot of CGI scripts fall into this category), you can write much more powerful programs if you have conditional expressions and looping mechanisms. Collectively, these are known as control structures. So you can also think of Perl as a control language.

But to have control, you have to be able to decide things, and to decide things, you have to know the difference between what's true and what's false.

What Is Truth?

We've bandied about the term truth,^[20] and we've mentioned that certain operators return a true or a false value. Before we go any further, we really ought to explain exactly what we mean by that. Perl treats truth a little differently than most computer languages, but after you've worked with it a while, it will make a lot of sense. (Actually, we hope it'll make a lot of sense after you've read the following.)

Basically, Perl holds truths to be self-evident. That's a glib way of saying that you can evaluate almost anything for its truth value. Perl uses practical definitions of truth that depend on the type of thing you're evaluating. As it happens, there are many more kinds of truth than there are of nontruth.

Truth in Perl is always evaluated in a ...