A subroutine encapsulates a routine so that other parts of the program can refer to it by its subroutine name.

Consider the addition algorithm; what if we needed to add various numbers several times in our program but did not want to create a separate piece of code for each instance of addition? If the algorithm were four or five lines long, it would be annoying to type the similar lines over and over again. Even worse, if you ended up changing the logic of the algorithm you would have to hunt down every occurrence and change each one. Maintaining such a program could become a nightmare, because many errors could arise if you forgot to change one of the lines of code in any of the duplicate routines or changed one of them incorrectly.

When faced with such a circumstance, a programmer can create a subroutine that can be used again and again by other parts of the program.

To create and use subroutines in Perl, we need three things: a subroutine reference, a subroutine identifier, and a parameter list of variables to pass to the subroutine. The & symbol precedes the name of the routine, telling Perl to look for the subroutine and call it. For example, &AddNumbers would direct Perl to execute the AddNumbers subroutine.

We also need to be able to send the subroutine some information. Specifically, we need to send the subroutine parameters that it will use to customize its output. If we want to sum 2 and 3, for example, we pass 2 and 3 to the subroutine using the following format:

    &AddNumbers(2,3);

The & marker tells Perl to look for the subroutine in the program in order to call it. However, the definition of the subroutine is marked off in the program using a sub marker. The code belonging to the routine is delimited with curly brackets ({}). The following example shows what the AddNumber subroutine definition would look like:

    sub AddNumbers
      {
      local($first_number, $second_number) = @_;
      print $first_number + $second_number;
     }

Note the third line above. We use the local keyword to make sure that the $first_number and $second_number variables will be considered local to only that subroutine. The subroutine will not affect any other variables that may be called $first_number or $second_number in other subroutines within the program.

In Perl, the @_ is a list of parameters that have been passed to the function. $first_number is set equal to the first parameter, and $second_number is set equal to the second parameter in the @_ list of parameters. If the routine is called by &AddNumbers(2,3), 2 and 3 will be assigned to $first_number and $second_number, respectively.

Whenever you want to add numbers, you can simply use the subroutine call &AddNumbers(x,y) instead of writing each addition individually. As a bonus, if you need to change the logic of the addition algorithm, you need only change it in the subroutine.