[perl] How can I debug a Perl script?

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?

perl -d your_script.pl args

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

I would also recommend using the Perl debugger.

However, since you asked about something like shell's -x have a look at the Devel::Trace module which does something similar.

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.

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.

The most effective debugging tool is still careful thought, coupled with judiciously placed print statements.

Brian Kernighan, "Unix for Beginners" (1979)

(And enhancing print statements with Data::Dumper)

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.

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

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