Declaring and Calling Subroutines

This section describes some important rules about declaring and calling user defined subroutines: parameters are passed as a list value stored in the special local variable @_; subroutines are normally called with their name prefixed with &.

Like many other programming languages, Perl allows you to define and call your own subroutines. But some of Perl subroutine rules are unusual comparing to other languages.

1. All parameters of a subroutine call are passed as a single list of scalars. If a list (array or hash) is used in a subroutine call as a parameter, it will be exploded into multiple parameters with its elements.

2. All parameters will be received as elements in a local array variable named as @_.

3. All parameters are passed as aliases. If the value of a parameter is modified inside a subroutine, the value of the corresponding variable in the calling code is also modified. But if the calling code is passing a literal as a parameter, then you can not modify the value of this parameter. You will get an execution error.

4. All subroutines return values. So in Perl, subroutines are really functions. If a subroutine is ended with no explicit return statement, the value of the last expression will be returned.

5. All return values are passed back to the caller as a single list of scalars. If a list (array or hash) is used as a return value, it will be exploded into multiple return values with its elements.

6. There are several ways to call a subroutine:

Table of Contents

 About This Book

 Perl on Linux Systems

 ActivePerl on Windows Systems

 Data Types: Values and Variables

 Expressions, Operations and Simple Statements

User Defined Subroutines

Declaring and Calling Subroutines - Example on Parameter List - Example on Parameters as Alias - Example on Return Values - Example on Calling Formats

 Perl Built-in Debugger

 Name Spaces and Perl Module Files

 Symbolic (or Soft) References

 Hard References - Addresses of Memory Objects

 Objects (or References) and Classes (or Packages)

 Typeglob and Importing Identifiers from Other Packages

 String Built-in Functions and Performance

 File Handles and Data Input/Output

 Open Files in Binary Mode

 Open Directories and Read File Names

 File System Functions and Operations

 Image and Picture Processing

 Using DBM Database Files

 Using MySQL Database Server

 Socket Communication Over the Internet

 XML::Simple Module - XML Parser and Generator

 XML Communication Model

 SOAP::Lite - SOAP Server-Client Communication Module

 Perl Programs as IIS Server CGI Scripts

 CGI (Common Gateway Interface)

 XML-RPC - Remote Procedure Call with XML and HTTP

 RPC::XML - Perl Implementation of XML-RPC

 Integrating Perl with Apache Web Server Module for Building Web Pages

 LWP::UserAgent and Web Site Testing

 Converting Perl Script to Executable Binary

 Managing Perl Engine and Modules on macOS


 Full Version in PDF/EPUB