How can I debug a Perl script?

Question

When I run a Perl script, how can I debug it? For example, in ksh I add the -x flag. But how I do the same in Perl?

Answer 1

If using an interactive debugger is OK for you, you can try perldebug.

Answer 2

If you want to do remote debugging (for CGI or if you don't want to mess output with debug command line), use this:

Given test:

use v5.14;
say 1;
say 2;
say 3;

Start a listener on whatever host and port on terminal 1 (here localhost:12345):

$ nc -v -l localhost -p 12345

For readline support use rlwrap (you can use on perl -d too):

$ rlwrap nc -v -l localhost -p 12345

And start the test on another terminal (say terminal 2):

$ PERLDB_OPTS="RemotePort=localhost:12345" perl -d test

Input/Output on terminal 1:

Connection from 127.0.0.1:42994

Loading DB routines from perl5db.pl version 1.49
Editor support available.

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

main::(test:2):    say 1;
  DB<1> n
main::(test:3):    say 2;
  DB<1> select $DB::OUT

  DB<2> n
2
main::(test:4):    say 3;
  DB<2> n
3
Debugged program terminated.  Use q to quit or R to restart,
use o inhibit_exit to avoid stopping after program termination,
h q, h R or h o to get additional info.
  DB<2>

Output on terminal 2:

1

Note the sentence if you want output on debug terminal

select $DB::OUT

If you are Vim user, install this plugin: dbg.vim which provides basic support for Perl.

Answer 3

Use Eclipse with EPIC: It gives you a nice IDE with debugging possibilities, including the ability to place breakpoints and the Perl Expression View for inspecting the value of variables.

Answer 4

perl -d your_script.pl args

is how you debug Perl. It launches you into an interactive gdb-style command line debugger.

Answer 5

To run your script under the Perl debugger you should use the -d switch:

perl -d script.pl

But Perl is flexible. It supplies some hooks, and you may force the debugger to work as you want

So to use different debuggers you may do:

perl -d:DebugHooks::Terminal script.pl
# OR
perl -d:Trepan script.pl

Look these modules here and here.

There are several most interesting Perl modules that hook into Perl debugger internals: Devel::NYTProf and Devel::Cover

And many others.

Answer 6

Note that the Perldebugger can also be invoked from the scripts shebang line, which is how I mostly use the -x flag you refer to, to debug shell scripts.

#! /usr/bin/perl -d