Learning Perl, Fourth Edition

Chapter 4. Subroutines

You’ve seen and used some of the built-in system functions, such as chomp, reverse, and print. But, as other languages do, Perl has the ability to make subroutines , which are user-defined functions.^[90] These let us recycle one chunk of code many times in one program.^[91] The name of a subroutine is another Perl identifier (letters, digits, and underscores, but it can’t start with a digit) occasionally with an optional ampersand (&) in front. There’s a rule about when you can omit the ampersand and when you cannot; you’ll see that rule by the end of the chapter. For now, we’ll use it every time it’s allowed, which is always a safe rule. We’ll tell you every place where it’s forbidden, of course.

The subroutine name comes from a separate namespace, so Perl won’t be confused if you have a subroutine called &fred and a scalar called $fred in the same program, though there’s no reason to do that under normal circumstances.

Defining a Subroutine

To define your own subroutine, use the keyword sub , the name of the subroutine (without the ampersand), and the indented block of code (in curly braces)^[92] that makes up the body of the subroutine, something like this:

    sub marine {
      $n += 1;  # Global variable $n

      print "Hello, sailor number $n!\n";
    }

Subroutine definitions can be anywhere in your program text, but programmers who come from a background of languages such as C or Pascal like to put them at the start of the file. Others may prefer to put them at the end of the ...