Perl debugger superpowers, part 2

In March I wrote The Perl debugger can be your superpower, introducing the step debugger as a better way to debug your Perl code rather than littering your source with temporary print statements or logging. I use the debugger all the time, and I’ve realized that some more techniques are worth covering.

Although I mentioned a caveat when debugging web applications, our apps at work all adhere to the Perl Web Server Gateway Interface (PSGI) specification and thus we can use tools like Test::WWW::Mechanize::PSGI or Plack::Test to run tests and debugging sessions in the same Perl process. (Mojolicious users can use something like Test::Mojo for the same effect.)

To demonstrate, let’s get started with something like this which tests that a given route (/say-hello) returns a certain JSON structure ({"message": "Hello world!"}):

#!/usr/bin/env perl

use Test::Most;
use Test::WWW::Mechanize::PSGI;
use JSON::MaybeXS;
use Local::MyApp; # name of app's main module

my $mech = Test::WWW::Mechanize::PSGI->new(
    # a Dancer2 app, so to_app returns a PSGI coderef
    app => Local::MyApp->to_app(),
lives_and {
    my $json = decode_json($mech->content);
    cmp_deeply( $json, {message => 'Hello world!'} );
} 'message is Hello world!';


All very fine and well, but what happens if that route starts returning a different message or worse, invalid output that causes decode_json to fail? Eventually, you’ll rewrite the test in the script to output the offending content when something goes wrong, but right now you want to suss out the root cause.

Debuggers have the concept of breakpoints, which are flags that tell the debugger to stop at a certain line of code and wait for instructions. We can set them while running the debugger with the b command or continue to a one-time breakpoint with the c command, or we can insert them into the code ourselves before running it through the debugger in the first place.

Add this line right after the lives_and { line:

$DB::single = 1;

This simulates having typed the s command in the debugger at that line, stopping execution at that point. Run our test with perl’s -d option, and then type c to continue to that breakpoint:

$ perl -d -Ilib t/test_psgi.t

Loading DB routines from version 1.60
Editor support available.

Enter h or 'h h' for help, or 'man perldebug' for more help.

[Local::MyApp:7170] core @2021-07-06 07:33:22> Built config from files: /Users/mgardner/Projects/blog/myapp/config.yml /Users/mgardner/Projects/blog/myapp/environments/development.yml in (eval 310)[/Users/mgardner/.plenv/versions/5.34.0/lib/perl5/site_perl/5.34.0/Sub/] l. 910
71: INIT { eval 'END { test2_set_is_end() }; 1' or die $@ }

  DB<1> c

ok 1 - GET /say-hello
15: my $json = decode_json($mech->content);


From here we can examine variables, set other breakpoints, or even execute arbitrary lines of code. Let’s see what became of that HTTP GET request:

DB<1> x $mech->content

0 '{"error":"Undefined subroutine &Local::MyApp::build_frog called at lib/Local/ line 11.\\n"}'


Aha, something has returned some different JSON indicating an error. Let’s look at the lines around (10–20) the offending line (11):

DB<2> f lib/Local/

  DB<3> l 10-20

10: my $method = 'build_frog';
11: $method->();
12 }
13: catch ($e) {
14: send_as JSON => {error => $e};
15 }
16: send_as JSON => {message => 'Hello world!'};
17: };
19 sub build_frob {
20: return;


Yep, a typo on line 11, and one that wasn’t caught at compile time since it’s generated at runtime.

Just to be sure (and to demonstrate some other cool debugger features), let’s set another breakpoint while in the debugger and then exercise that route again. Then we’ll check that $method variable against the list of available methods in the Local::MyApp package.

DB<4> b 11

  DB<5> $mech->get('/say-hello')

11: $method->();

  DB<<6>> x $method

0 'build_frog'

  DB<<7>> m Local::MyApp

No doubt about it, that variable is being set incorrectly.

Quit out of the debugger with the q command, make the fix (we probably want errors to give something other than an HTTP 200 OK while we’re at it), and re-run the test:

$ perl -Ilib t/test_psgi.t

[Local::MyApp:8277] core @2021-07-06 07:48:36> Built config from files: /Users/mgardner/Projects/blog/myapp/config.yml /Users/mgardner/Projects/blog/myapp/environments/development.yml in (eval 309) l. 910
Name "DB::single" used only once: possible typo at t/test_psgi.t line 13.
[Local::MyApp:8277] core @2021-07-06 07:48:36> looking for get /say-hello in /Users/mgardner/.plenv/versions/5.34.0/lib/perl5/site_perl/5.34.0/Dancer2/Core/ l. 35
[Local::MyApp:8277] core @2021-07-06 07:48:36> Entering hook in (eval 274) l. 1
[Local::MyApp:8277] core @2021-07-06 07:48:36> Entering hook in (eval 274) l. 1
[Local::MyApp:8277] core @2021-07-06 07:48:36> Entering hook in (eval 274) l. 1
[Local::MyApp:8277] core @2021-07-06 07:48:36> Entering hook in (eval 274) l. 1
ok 1 - GET /say-hello
ok 2 - message is Hello world!

Note that warning about leaving $DB::single in there. While harmless, it’s a good reminder to remove such lines from your code so that they don’t surprise you or your teammates during future debugging sessions.

And that’s it. Note that because we’re using PSGI, we were able to set breakpoints in our web app code itself and the debugger stopped there and enabled us to have a look around. And as you’ve seen, once you’re at a breakpoint you can switch to different files, add/remove more breakpoints, run arbitrary code, and more. The perldebug documentation page has all the details.

Happy debugging! For your reference, here’s the full app module and test script used in this article: