Mastering Perl

Chapter 12. Detecting and Reporting Errors

Several things may go wrong in any program, including problems in programming, bad or missing input, unreachable external resources, and many other things. Perl doesn’t have any built-in error handling. It knows when it couldn’t do something, and it can tell me about errors, but it’s up to me as the Perl programmer to ensure that my program does the right thing, and when it can’t, try to do something useful about it.

Perl Error Basics

Perl has four special variables it uses to report errors: $!, $?, $@, and $^E. Each reports different sorts of errors. Table 12-1 shows the four variables and their descriptions, which are also in perlvar.

Table 12-1. Perl’s special error-reporting variables

Variable	English	Description
$!	$ERRNO and $OS_ERROR	Error from an operating system or library call
$?	$CHILD_ERROR	Status from the last wait() call
$@	$EVAL_ERROR	Error from the last eval()
$^E	$EXTENDED_OS_ERROR	Error information specific to the operating system

Operating System Errors

The simplest errors occur when Perl asks the system to do something, but the system can’t or doesn’t do it for some reason. In most cases the Perl built-in returns false and sets $! with the error message. If I try to read a file that isn’t there, open returns false and puts the reason it failed in $!:

open my( $fh ), '<', 'does_not_exist.txt'
        or die "Couldn't open file! $!";

The Perl interpreter is a C program, and it does its work through the library of C functions it’s built upon. The value ...