The Perl interpreter distinguishes between warnings and errors. Warnings are messages that the Perl interpreter prints to STDERR (or to Apache's error log under mod_perl). These messages indicate that Perl thinks there is a problem with your code, but they do not prevent the code from running. Errors are output in the same way as warnings, but the program terminates after an error. For example, errors occur if your code uses invalid syntax. If a die( ) occurs outside of any exception-handling eval, it behaves just like an error, with a message being output and program execution terminating.

For someone new to Perl programming, the warning and error messages output by Perl can be confusing and worrysome. In this section we will show you how to interpret Perl's messages, and how to track down and solve the problems that cause them.