Perl::Critic is a lint-like tool for Perl. It uses static analysis (it does not run your code) and provides a list of places where the code violates one of the policy rules.

Originally based on the book Perl Best Practices written by Damian Conway, the module is extendable and customizable. You, or your team can set up your own standard, and use Perl::Critic to enforce it.

The Perl::Critic module comes with a command line tool called perlcritic, but you can also integrate the use of Perl::Critic in your unit-tests via the Test::Perl::Critic wrapper.

As an example, let's see the results of the default, the most gentle level of criticism of a script I saw recently. Let's also add some commentary.

Code before strictures are enabled

$ perlcritic example.pl
Bareword file handle opened at line 1, column 1.  See pages 202,204 of PBP.  (Severity: 5)
Two-argument "open" used at line 1, column 1.  See page 207 of PBP.  (Severity: 5)
Code before strictures are enabled at line 1, column 1.  See page 429 of PBP.  (Severity: 5)

This report was given for the following code: 

open FH, $filename or die;

Let's start with the third entry. It complains about that fact that there is no use strict;. As I recommend, one should always use strict!

So I changed the code and ran perlcritic again. 

use strict;
open FH, $filename or die;

Bareword file handle and Two-argument "open"

This time the output was this: 

Bareword file handle opened at line 2, column 1.  See pages 202,204 of PBP.  (Severity: 5)
Two-argument "open" used at line 2, column 1.  See page 207 of PBP.  (Severity: 5)

A bareword is a series of characters (like the FH in the above code snippet) without quotes around them, or without a sigil in-front of them. They can be used in some places, but in file-handles they are not recommended any more.

The recommendation is to always use lexical variables for file-handles and always use 3-parameter open.

Based on the recommendation on how to improve perl script I've updated the script to have this code: 

use strict;
open my $fh, '<:encoding(UTF-8)', $filename or die;

That was the gentle level of Perl::Critic. Let's make it a bit more stern by running perlcritic --stern example.pl:

Exclude specific policy violations

This is the output I got: 

Code not contained in explicit package at line 1, column 1.  Violates encapsulation.  (Severity: 4)
Close filehandles as soon as possible after opening them at line 2, column 1.  See page 209 of PBP.  (Severity: 4)
Module does not end with "1;" at line 2, column 1.  Must end with a recognizable true value.  (Severity: 4)
Code before warnings are enabled at line 2, column 1.  See page 431 of PBP.  (Severity: 4)

Out of these 4 violations two relate to the fact that we write a script and not a module. We don't really want to care about those and so we would like to turn those off. There are a number of ways to do that, one of them is to provide the names of the rules on the command line with the --exclude flag.

In order to find out the names we need to exclude we can run perlcritic --stern --verbose 8 examples.pl that will display the following output: 

[Modules::RequireExplicitPackage] Code not contained in explicit package at line 1, column 1.  (Severity: 4)
[InputOutput::RequireBriefOpen] Close filehandles as soon as possible after opening them at line 2, column 1.  (Severity: 4)
[Modules::RequireEndWithOne] Module does not end with "1;" at line 2, column 1.  (Severity: 4)
[TestingAndDebugging::RequireUseWarnings] Code before warnings are enabled at line 2, column 1.  (Severity: 4)

From that we create the following command:

perlcritic --stern --exclude RequireExplicitPackage --exclude RequireEndWithOne example.pl

will print this: 

Close filehandles as soon as possible after opening them at line 2, column 1.  See page 209 of PBP.  (Severity: 4)
Code before warnings are enabled at line 2, column 1.  See page 431 of PBP.  (Severity: 4)

Close filehandles as soon as possible after opening them

tells use that we should call close $fh soon after calling open.

Code before warnings are enabled

tells us to add use warnings; at the beginning of the code just as it is explained in the article that asks you to always use warnings

More levels of perlcritic

The perlcritic command line tool actually has 5 levels of severity. So far we tried level 5 and 4 corresponding to the words "gentle" and "stern". The other 3 levels are "harsh", "cruel", and "brutal" creating a growing level of pain. You can try them and see what kind of violations they report on your code.