Perl Pocket Reference, 4th Edition

Chapter 12. Subroutines

Subroutines need to be declared, i.e., specified how they should be called, and defined, i.e., specified what they should do when called.

sub name [ ( proto ) ] [ attributes ]

Declares name as a subroutine, optionally specifying the prototype and attributes. Declaring a subroutine is optional, but allows the subroutine to be called just like Perl’s built-in operators.

sub [ name ] [ ( proto ) ] [ attributes ] block

Defines subroutine name, with optional prototype and attributes. If the subroutine has been declared with a prototype or attributes, the definition should have the same prototype and attributes. When name is omitted, the subroutine is anonymous and the definition returns a reference to the code.

When a subroutine is called, the statements in block are executed. Parameters are passed as a flat list of scalars as array @_. The elements of @_ are aliases for the scalar parameters. The call returns the value of the last expression evaluated. wantarray can be used to determine the context in which the subroutine was called.

Subroutines that have an empty prototype and do nothing but return a fixed value are inlined, e.g., sub PI() { 3.1415 }.

attributes are introduced with a : (colon). The following attributes are currently implemented:

method	The subroutine is a method.
locked	Lock this subroutine against concurrent access.
lvalue	The subroutine returns a variable that can be assigned to.

There are several ways to call a subroutine.

name ( [ parameters ] )

The ...