Debugging Perl Scripts

(Up-to-date source of this post.)

When there's a problem while writing a Perl program the thing I do often is to print out the contents of a variable. There are two ways I usually do it.

If the the variable is not complex (i.e. it's a simple scalar, array or hash variable) I use the print function:

#!/usr/bin/perl
use strict;
use warnings;

# A number between 1 and 100 (both included)
my $secret = int( 1 + rand 100 );

# If DEBUG environment variable set, print out the secret number
print "The secret number is $secret"
    if $ENV{DEBUG};

Then I just set the environment variable while running the script:

$ DEBUG=1 ./script.pl
The secret number is 3

In case I'm dealing with a more complicated data structure, like the $server variable below, I use Data::Dumper (which outputs valid Perl code):

#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

my $name = 'foo';

my $server = {
    'server' => {
        'imageRef'  => '8a3a9f96-b997-46fd-b7a8-a9e740796ffd',
        'flavorRef' => '2',
        'name'      => $name,
        'metadata'  => { 'My Server Name' => 'Ubuntu 12.10 (Quantal Quetzal)' },
    }
};

print Dumper $server;

The Perl Debugger

Sometimes I need something more than just printing out the variable contents. In that case I reach for the Perl debugger:

$ perl -d prog.pl

It shows each line of code before it executes it. The most common commands are these:

• h -- help
• s -- single-step the program
• x -- dump a variable value (for complex data use x \%href or even x sort keys %hash)
• n -- step over a subroutine
• q -- quit the debugger

To have the command line history within the Perl debugger, you need to install:

• Debian/Ubuntu packages: libncurses-dev libreadline-dev
• Perl module: Term::ReadLine::Gnu

More

• Basic debugging checklist
• Debugging Perl with hdb (GUI debugger)