Perl in a Nutshell

Chapter 6. Debugging

Of course everyone writes perfect code on the first try, but on those rare occasions when something goes wrong and you’re having trouble with your Perl script, there are several things you can try:

• Run the script with the -w switch, which prints warnings about possible problems in your code.

• Use the Perl debugger.

• Use another debugger, or a profiler such as the Devel::DProf module.

The major focus of this chapter is the Perl debugger, which provides an interactive Perl environment. The chapter also describes the use of the DProf module and the dprofpp program that comes with it; together they can provide you with a profile of your Perl script. If you’ve ever used any debugger, and you understand concepts such as breakpoints and backtraces, you’ll have no trouble learning to use the Perl debugger. Even if you haven’t used another debugger, the command descriptions and some experimenting should get you going.

The Perl Debugger

To run your script under the Perl source debugger, invoke Perl with the -d switch:

perl -d myprogram

This works like an interactive Perl environment, prompting for debugger commands that let you examine source code, set breakpoints, get stack backtraces, change the values of variables, etc. If your program takes any switches or arguments, you must include them in the command:

perl -d myprogram myinput

In Perl, the debugger is not a separate program as it is in the typical compiled environment. Instead, the -d flag tells the compiler to insert ...