If you look at a Perl program from a very high level, it is made of statements. Statements are a complete unit of instruction for the computer to process. The computer executes each statement it sees-in sequence-until a jump or branch is processed.
Statements can be very simple or very complex. The simplest statement is this
which is a numeric literal followed by a semicolon. The semicolon is very important. It tells Perl that the statement is complete. A more complicated statement might be
$bookSize = ($numOfPages >= 1200 ? "Large" : "Normal");
which says if the number of pages is 1,200 or greater, then assign "Large" to $bookSize; otherwise, assign "Normal" to $bookSize.
In Perl, every statement has a value. In the first example, the value of the statement is 123.
In the second example, the value of the statement could be either "Large" or "Normal" depending on the value of $numOfPages. The last value that is evaluated becomes the value for the statement.
Like human language in which you put statements together from parts of speech-nouns, verbs, and modifiers-you can also break down Perl statements into parts. The parts are the literals, variables, and functions you have already seen in the earlier chapters of this book.
Expressions are a sequence of literals, variables, and functions connected by one or more operators that evaluate to a single value-scalar or array. An expression can be promoted to a statement by adding a semicolon. This was done for the first example earlier. Simply adding a semicolon to the literal made it into a statement that Perl could execute.
Expressions may have side effects, also. Functions that are called can do things that are not immediately obvious (like setting global variables) or the pre- and post-increment operators can be used to change a variable's value.
Let's take a short diversion from our main discussion about statements
and look at expressions in isolation. Then we'll return to statements
to talk about statement blocks and statement modifiers.