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Perl | Breakpoints of a Debugger

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Controlling program execution in Perl can be done by telling the debugger to execute up to a certain specified point in the program, called a breakpoint. These breakpoints enable the user to divide the program into sections and look for the errors. Following are some commands in a debugger which are used to create such breakpoints and the commands that are into execution until a breakpoint is reached.  


  b-command is used to set a breakpoint in a program. Whenever a specified line is about to be executed, this command tells the debugger to halt the program. For example, below given command tells the debugger to halt when it is about to execute line 12:

DB<13> b 12

(The debugger will return Line 12 is not breakable, if the line is unbreakable) Note : There can be any number of breakpoints in a program. The debugger will halt the program execution whenever it is about to execute any of the statement which contains a breakpoint. b-command accepts subroutine names as well:

DB<15> b valuedir

This creates a breakpoint at the very first executable statement of the subroutine valuedir. b-command can also be used to halt a program only when a specified condition meets. For example, below mentioned command tells the debugger to halt when it is about to execute line 12 and the variable $vardir is equal to the null string:

DB<15> b 12 ($vardir eq "")

Any legal Perl conditional expression can be specified with the b statement. Note : For a multiline statement, a breakpoint can be set at any of those lines. For example:

16: print ("Geeks", 
17: " for Geeks here");

Here, a breakpoint can be set at line 16, but not line 17.  

The c-Command

  c-command is used to instruct the debugger to continue the debugging process until it reaches a breakpoint or the end of the program.

DB<15> c

main::(debugtest:12):                  $vardir =~ a/^\a+|\a+$//h;


When the debugger is about to reach the line 12-at which our breakpoint is set-debugger halts the program and displays the line as the debugger always displays the line which is about to be executed. Here, a prompt is generated by the debugger for another debugging command. This prompt is to enable you to further start the execution process one statement at a time using either n or s, continue the execution process using c, more breakpoints can be set using b, or any other debugging operation can be performed. A temporary(one-time-only) breakpoint can be set with the use of c-command by supplying a line number:

DB<15> c 12

main::(debugtest:12):                      &readsubdirs($vardir);

The argument 12 specified with the c-command tells the debugger to set a temporary breakpoint at line 12 and then continue execution. When line 12 is reached, debugger halts the execution process, displays the line, and removes the breakpoint. When the user wants to skip a few lines of the program and don’t want to waste the execution time by going through the program one statement at a time, c-command is used to define a temporary breakpoint. Using c also ensures that there is no need to define a breakpoint using b and further deleting it using d-command.  

The L Command and Breakpoints

  L-command is used to provide a list of all the breakpoints used in the program. This command lists the last few lines executed, the current line into execution, the breakpoints which were defined and the conditions under which breakpoints were into effect.

DB<18> L

4:      $count = 0;

5:      $vardir = "";

6:      while (1) {

8:              if ($vardir eq "") {

11:                      $vardir =~ a/^\a+|\a+$//h;

  break if (1)

Here, the program has executed lines 4-8, and a breakpoint is set for line 11. (Line 7 is not listed because it is a comment) Breakpoints can be distinguished from executed lines by looking for the conditional expressions, which are immediately after the breakpoint. Here, the conditional expression is set to (1), which indicates that the breakpoint is always effective.  

The d and D Commands

  When the job of a breakpoint is finished, it can be deleted by using the d-command.

DB<16> d 12

Above mentioned command tells the debugger to delete the breakpoint which was set at line 12. If a breakpoint is not specified to be deleted, the debugger assumes that a breakpoint is defined for the next line which is to be executed and deletes it by itself.

main::(debugtest:12):                      &readsubdirs($vardir);

DB<17> d

Here, line 12 is the next line which is to be executed, so the debugger automatically deletes the breakpoint placed at line 12. D-command is used to delete all the breakpoints set in the program.

DB<18> D

Above command deletes, all the breakpoints defined with the b command.

In Perl, you can use the built-in debugger to step through your code and examine variables and data structures. Breakpoints are a key feature of the debugger that allow you to stop the execution of your program at specific lines of code, so you can examine the state of your program at that point.

To set a breakpoint in the Perl debugger, you can use the b command followed by the line number or subroutine name where you want to stop the execution of your program. For example, if you want to set a breakpoint at line 10 of your Perl script, you can use the following command in the debugger:

b 10

Here’s an example of how to use breakpoints in the Perl debugger:


# your code here
use strict;
use warnings;
my $x = 10;
my $y = 20;
sub add_numbers {
    my ($a, $b) = @_;
    my $sum = $a + $b;
    return $sum;
my $result = add_numbers($x, $y);
print "Result: $result\n";


Result: 30

The debugger will stop at line 8 and you can examine the values of $x and $y using the p command. You can also step through the rest of the code using the n command and examine the value of $sum using the p command again. When you’re finished, you can exit the debugger using the q command.

The use of breakpoints in the Perl debugger has several advantages and disadvantages:


  • Allows you to stop the execution of your program at specific lines or subroutines, which makes it easier to debug your code.
  • Provides an interactive interface for examining variables and data structures, which can help you identify problems in your code more quickly.
  • Can help you understand how your code is executing, which can be useful for optimizing performance or improving readability.
  • Offers a convenient way to trace the flow of your program, which can be especially helpful when working with large or complex code bases.


  • Can slow down the execution of your program, particularly if you set many breakpoints or have a large code base.
  • Can be difficult to use effectively if you’re not familiar with the debugger or with the specific code you’re working with.
  • May not always identify the root cause of a problem, particularly if the problem is caused by a complex interaction between different parts of your code.
  • Can be time-consuming to use, particularly if you have to step through many lines of code to find a problem.

Some important points to keep in mind when working with Perl:

  1. Perl is a powerful scripting language that is particularly well-suited for text processing, system administration, and web development.
  2. Perl supports both procedural and object-oriented programming paradigms, and provides a wide range of built-in functions and modules to facilitate development.
  3. Perl’s syntax is flexible and expressive, which makes it easy to write concise and readable code.
  4. Perl has a large and active user community, which means that there are many resources available for learning and troubleshooting.

Some recommended books for learning Perl are:

  • “Learning Perl” by Randal L. Schwartz, brian d foy, and Tom Phoenix
  • “Intermediate Perl” by Randal L. Schwartz, brian d foy, and Tom Phoenix
  • “Programming Perl” by Larry Wall, Tom Christiansen, and Jon Orwant
  • “Modern Perl” by chromatic\
  • “Effective Perl Programming” by Joseph N. Hall and Joshua A. McAdams
  • These books provide a comprehensive introduction to Perl and cover both basic and advanced topics in the language.

Last Updated : 13 Apr, 2023
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