Learning Perl, 3rd Edition

Chapter 4. Subroutines

System and User Functions

We’ve already seen and used some of the builtin 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.^[1] These let us recycle one chunk of code many times in one program.^[2]

The name of a subroutine is another Perl identifier (letters, digits, and underscores, but 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; we’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. And we’ll tell you every place where it’s forbidden, of course.

That 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^[3] block of code (in curly braces) 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. ...