Perl in a Nutshell, 2nd Edition

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 source information into the parse trees it’s about to hand off to the interpreter. This means your code must compile correctly for the debugger to work on it—the debugger won’t run until you have fixed all compiler errors.

After your code has compiled, and the debugger has started up, the program halts right before the first runtime executable statement (see Section 6.3 regarding compile time statements) and waits for you to enter a debugger command. Whenever the debugger halts and shows you a line of code, it always displays the line it’s about to execute, rather than the one it has just executed.

Any command not recognized by the debugger is directly executed as Perl code in the current package. To be recognized by the debugger, the command must start at the beginning of the line; otherwise, the debugger assumes it’s for Perl.