Programming Perl, 3rd Edition

Chapter 20. The Perl Debugger

First of all, have you tried the use warnings pragma?

If you invoke Perl with the -d switch, your program will be run inside the Perl debugger. This works like an interactive Perl environment, prompting for debugger commands that let you examine source code, set breakpoints, dump out your function-call stack, change the values of variables, and so on. Any command not recognized by the debugger is directly executed (using eval) as Perl code in the package of the code currently being debugged. (The debugger uses the DB package for its own state information, to avoid trampling yours.) This is so wonderfully convenient that people often fire up the debugger just to test out Perl constructs interactively. In that case, it doesn't matter what program you tell Perl to debug, so we'll choose one without much meaning:

% perl -de 42

In Perl, the debugger is not a program completely separate from the one being debugged, the way it usually is in a typical programming environment. Instead, the -d flag tells the compiler to insert source information into the parse trees it's about to hand off to the interpreter. That means your code must first compile correctly for the debugger to work on it. If that is successful, the intrepreter preloads a special Perl library file containing the debugger itself.

% perl -d /path/to/program

The program will halt immediately before the first run-time executable statement (but see Section 20.1 regarding compile-time statements) ...