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Chapter 10. Subroutines


Accessing Subroutine Arguments
Making Variables Private to a Function
Creating Persistent Private Variables
Determining Current Function Name
Passing Arrays and Hashes by Reference
Detecting Return Context
Passing by Named Parameter
Skipping Selected Return Values
Returning More Than One Array or Hash
Returning Failure
Prototyping Functions
Handling Exceptions
Saving Global Values
Redefining a Function
Trapping Undefined Function Calls with AUTOLOAD
Nesting Subroutines
Writing a Switch Statement
Program: Sorting Your Mail

W. H. Auden, "Three Songs for St Cecilia's Day"

Composing mortals with immortal fire.

10.0. Introduction

To avoid the dangerous practice of copying and pasting code, larger programs reuse chunks of code as subroutines and functions. We'll use the terms subroutine and function interchangeably because Perl doesn't distinguish between the two. Even object-oriented methods are just subroutines that are called using a special syntax, described in Chapter 13.

A subroutine is declared with the sub keyword. Here's a simple subroutine definition:

sub hello { 
    $greeted++;          # global variable 
    print "hi there!\n";

The typical way of calling that subroutine is:

hello( );                 # call subroutine hello with no arguments/parameters

Because Perl compiles your program before executing it, it doesn't matter where subroutines are declared. Definitions don't have to be in the same file as your main program. They can be pulled in from other files using the do, require, or use operators, as described in Chapter 12. They can even be created on the fly using eval or AUTOLOAD, or generated using closures, which can act as function templates.

If you are familiar with other programming languages, several characteristics of Perl's functions may surprise you if you're unprepared for them. Most recipes in this chapter illustrate how to be aware of—and to take advantage of—these properties.

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