Declaring and Invoking Functions

A function (or subroutine) in Perl is a discrete, encapsulated behavior unit. A program is a collection of little black boxes where the interaction of these functions governs the program’s control flow. A function may have a name. It may consume incoming information. It may produce outgoing information.

Functions are a prime mechanism for organizing code into similar groups, identifying individual pieces by name, and providing reusable units of behavior.

Declaring functions

We use the sub built-in to declare a function:

sub greet_me { ... }

Now, we can invoke greet_me() from anywhere within our program.

Just as we may declare a lexical variable but leave its value undefined, we may declare a function without defining it. A forward declaration tells Perl to record that a named function exists. We may define it later:

sub greet_sun;

Invoking functions

We use postfix parentheses to invoke a named function. Any arguments to the function may go within the parentheses. While these parentheses are not strictly necessary for these examples, even with strict enabled, they provide clarity to human readers and Perl’s parser. When in doubt, use them.