Beginners	Built-In Functions	Advanced	CGI Applications
Introduction Syntax Operators Variables Scope Data Types Flow Control Subroutines RegEx	Arithmetic Strings Arrays Hashes Date/Time Printing	Files Directories Databases Reports RegEx Exceptions System Calls Modules One Liners Reference	Unix Forms CGI Cookies

Exception Functions Summary
When Perl detects an error it attempts to work around the error and continue execution. If it cannot resolve the problem, Perl displays an error message and stops execution of the script.

Alternately, the following three functions may be used to modify how errors are handled. Generally, these functions allow a custom error message to be displayed in addition to, or in place of, the default Perl error messages. The functions also allow the program to decide whether to terminate the program.

Error Handling	die, eval, warn
Special Variables	$!, $@

Exception Functions Syntax
Here are the basic ways in which the die function can be used to respond to an error message.

    1 open (INFILE, "input.txt");
    2 open (INFILE, "input.txt") || die;
    3 open (INFILE, "input.txt") || die "Bad file";
    4 open (INFILE, "input.txt") || die "Bad file \n";
    5 open (INFILE, "input.txt") || die "Bad file \n $!";

The effect of each lines is discussed below, on the assumption that the file 'input.txt' is not found.

• Line 1 - error not acknowledged (die not used)
  Perl gives no error message and the program continues to run. However, the error number and error message is stored in the special variable $!.

• Line 2 - standard Perl error message
  Perl terminates the program and the following output is displayed.

          Died at test.pl line 1
      

• Line 3 - custom plus standard error messages
  Perl terminates the program and the following output is displayed (a combination of the custom message and location of the error).

          Bad file at test.pl line 1
      

• Line 4 - custom error message only
  Perl terminates the program and the following output is displayed. The \n tells Perl not to display the standard error message nor the location of the error.

          Bad file
      

• Line 5 - custom plus standard error messages
  Perl terminates the program and the following output is displayed. The special variable $! provides the standard error message.

          Bad file
          No such file or directory at test.pl line 1
      

If the warn function is used in place of the die function, the same messages are displayed but program execution does not stop.

Eval Function Syntax
Perl also provides a way to trap errors - to prevent them from terminating the program and to allow the programmer to decide what action to take. Consider the following lines of code.

     1  $a/$b 
     2  eval{$a/$b}; 
     3  if ($@) {print "Error occurred: $@. Program will continue!"}

Line 1 will crash a program is $b has a value of zero.

Line 2 shows that wrapping the code in a block labelled 'eval' will trap the error and Perl will not terminate the program. The error message will be placed in the special variable $@. If there was no error, then $@ will be empty.

Exception Function Reference
Here's a quick reference of the available exception functions, in alphabetical order.

• die - return an error message, then raise an exception

      die LIST
      

• eval - catch exceptions

      eval EXPR
      

• warn - return an error message (does not raise an exception)

      warn LIST
      

• $! - returns either the error number or the error message of the last operations, depending on whether used in scalar or string context. It only contains valid data immediately after a failed system request.

• $@ - the syntax error or routine error message from the last eval, If set, either the compilation failed, or the die function was executed within the code of the eval.

If you have any suggestions for additions to these tutorials, please let me know.