Figuring out why your program is broke can be hard. If you've added print statements and turned up the logging and still can't find the problem, you might reach for the command-line debugger that comes with perl. In this presentation, I'll show off an alternative. Devel::hdb is a graphical debugger for Perl that runs inside a web browser with a simpler interface. I'll also briefly cover how to write your own debugging tools with Devel::Chitin, and preview a new feature I've been working on.

1. Debugging your Perl programs

2. Debugging your Perl programs
Methods
● print

3. Debugging your Perl programs
Methods
● print
● warn

4. Debugging your Perl programs
Methods
● print
● warn
● die

5. Debugging your Perl programs
Methods
● print
● warn
● die
● Logging (print to file, Log4Perl, etc)

6. Debugging your Perl programs
Methods
● print
● warn
● die
● Logging (print to file, Log4Perl, etc)
● tie() variables

7. Debugging your Perl programs
Methods
● print
● warn
● die
● Logging (print to file, Log4Perl, etc)
● tie() variables
● Data::Dumper, Data::Dump

8. Debugging your Perl programs
Methods
● print
● warn
● die
● Logging (print to file, Log4Perl, etc)
● tie() variables
● Data::Dumper, Data::Dump
● Hook::LexWrap

9. Debugging your Perl programs
> perl -d try.pl
Loading DB routines from perl5db.pl version 1.39_10
Editor support available.
Enter h or 'h h' for help, or 'man perldebug' for more help.
main::(try2.pl:4): print "hello worldn";
DB<1> _

10. Debugging your Perl programs
DB<1> h
List/search source lines: Control script execution:
l [ln|sub] List source code T Stack trace
- or . List previous/current line s [expr] Single step [in expr]
v [line] View around line n [expr] Next, steps over subs
f filename View source in file <CR/Enter> Repeat last n or s
/pattern/ ?patt? Search forw/backw r Return from subroutine
M Show module versions c [ln|sub] Continue until position
Debugger controls: L List break/watch/actions
o [...] Set debugger options t [n] [expr] Toggle trace [max depth] ][trace expr]
<[<]|{[{]|>[>] [cmd] Do pre/post-prompt b [ln|event|sub] [cnd] Set breakpoint
! [N|pat] Redo a previous command B ln|* Delete a/all breakpoints
H [-num] Display last num commands a [ln] cmd Do cmd before line
= [a val] Define/list an alias A ln|* Delete a/all actions
h [db_cmd] Get help on command w expr Add a watch expression
h h Complete help page W expr|* Delete a/all watch exprs
|[|]db_cmd Send output to pager ![!] syscmd Run cmd in a subprocess
q or ^D Quit R Attempt a restart
Data Examination: expr Execute perl code, also see: s,n,t expr
x|m expr Evals expr in list context, dumps the result or lists methods.
p expr Print expression (uses script's current package).
S [[!]pat] List subroutine names [not] matching pattern
V [Pk [Vars]] List Variables in Package. Vars can be ~pattern or !pattern.
X [Vars] Same as "V current_package [Vars]". i class inheritance tree.
y [n [Vars]] List lexicals in higher scope <n>. Vars same as V.
e Display thread id E Display all thread ids.
For more help, type h cmd_letter, or run man perldebug for all docs.
DB<1>

11. Debugging your Perl programs
> perl -d:ptkdb try.pl

13. Debugging your Perl programs
> perl -d:hdb try.pl
Debugger pid 87264 listening on
http://127.0.0.1:8080/debugger-gui

15. Debugging your Perl programs
> perl -d:hdb try.pl
HTTP::Server::PSGI (part of Plack)
jQuery
Twitter Bootstrap
Handlebars
Devel::Chitin

16. Demo!
> perl -d:hdb demo_program.pl
● Single step, over, etc
● Watch variables
● Breakpoints
● Stack
● Mouseover variables in other frames
● Breakpoints/actions
● Fork
● Uncaught exceptions

18. Trace and Follow
> perl -d:hdb=trace:tracefile trace_follow.pl 1

19. Trace and Follow
> perl -d:hdb=trace:tracefile trace_follow.pl 1
Writing execution trace to tracefile...
program is running
Everything is working ok
Doing some more stuff...
Exiting
> _

20. Trace and Follow
> perl -d:hdb=trace:tracefile trace_follow.pl 1
Writing execution trace to tracefile...
program is running
Everything is working ok
Doing some more stuff...
Exiting
> perl -d:hdb=follow:tracefile trace_follow.pl 0
Debugger pid 87677 listening on
http://127.0.0.1:8080/debugger-gui

22. Devel::Chitin
● Extracted debugger-specific code
● Object-oriented interface
● Multiple debugger modules

23. Devel::Chitin – Basic control
$db->attach();
$db->detach();

24. Devel::Chitin – Basic control
$db->attach();
$db->detach();
$db->step; $db->stepover(); $db->stepout();
$db->continue();
$db->trace($flag);

25. Devel::Chitin – Info
$bool = $db->is_loaded($filename);
$arrayref = $db->file_source($filename);
@names = $db->loaded_files();
$stack = $db->stack();
$loc = $db->current_location();

26. Devel::Chitin – Events
$db->init();
$db->notify_stopped($loc);
$db->notify_trace($loc);
$db->notify_fork_parent($loc, $pid);
$db->notify_fork_child($loc);
$db->notify_program_terminated($loc);
$db->notify_uncaught_exeption($exc);

27. Simple Debugger Demo
perl -d:SimpleDebugger trace_follow.pl

28. Simple Debugger Demo
package Devel::SimpleDebugger;
use base qw(Devel::Chitin);
Devel::SimpleDebugger->attach();
sub notify_stopped {
my($self, $location) = @_;
my $file_lines = $self->file_source($location->filename);
printf("Stopped at %s:%d >> %s",
$location->filename,
$location->line,
$file_lines->[ $location->line ]);
scalar(<STDIN>);
$self->step;
}
1;

29. Simple Profiler Demo
perl -d:SimpleProf trace_follow.pl

30. Simple Profiler Demopackage Devel::SimpleProf;
use strict;
use warnings;
use base qw(Devel::Chitin);
use Time::HiRes;
Devel::SimpleProf->attach();
Devel::SimpleProf->trace(1);
my($last_location, $last_time) = (0, Time::HiRes::time);
sub notify_trace {
my $now = Time::HiRes::time();
my($self, $location) = @_;
if ($last_location) {
my $diff = $now - $last_time;
printf("%s:%d took %0.3f msn",
$last_location->filename,
$last_location->line,
($now - $last_time)*1000);
}
}
sub notify_trace_resumed {
my($self, $location) = @_;
$last_location = $location;
$last_time = Time::HiRes::time;
}
1;

31. Both at the same time
perl -d:SimpleProf -MDevel::SimpleDebugger
trace_follow.pl

32. Devel::Chitin – new stuff
my $perl = $db->next_statement()
Deparses the OpTree at the stopped location
back into Perl code

33. Devel::Chitin – new stuff
my $perl = $db->next_statement()
> perl -d:Chitin -MShowNext deparse_demo.pl

34. Sources
Devel::hdb
● https://metacpan.org/pod/Devel::hdb
● https://github.com/brummett/Devel-hdb
Devel::Chitin
● https://metacpan.org/pod/Devel::Chitin
● https://github.com/brummett/Devel-Chitin
Demo code
● https://github.com/brummett/Devel-hdb-talk