Learning Perl, 5th Edition

Chapter 4. Subroutines

You’ve already seen and used some of the built-in system functions, such as chomp, reverse, print, and so on. But, as other languages do, Perl has the ability to make subroutines, which are user-defined functions.^[10] These let us recycle one chunk of code many times in one program.^[11] The name of a subroutine is another Perl identifier (letters, digits, and underscores, but it can’t start with a digit) with a sometimes-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 just use it every time that it’s not forbidden, 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—although 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), then the indented block of code (in curly braces),^[12] which 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 like C or Pascal like to put them at the start of the file. Others may prefer to ...