Perl Language

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Introduction

Perl is a pragmatic, text-friendly language.

  • Scripting language
  • Strong text processing
  • Dynamic typing
  • Multi-paradigm (procedural, OO, functional style)
#!/usr/bin/env perl
use strict;
use warnings;

print "Hello World\n";

VS Code and Perl

Package Installation

sudo apt update

# Core build + Perl tooling
sudo apt install -y build-essential git perl perl-dbg cpanminus

# Common native libs used by CPAN modules
sudo apt install -y libssl-dev libreadline-dev zlib1g-dev libffi-dev libyaml-dev

# Postgres client/dev + DBI drivers
sudo apt install -y libpq-dev libdbi-perl libdbd-pg-perl

# Web framework (optional)
sudo apt install -y libmojolicious-perl

# If you build XS modules that need libperl/libaio
sudo apt install -y libperl-dev libaio-dev

Perl local::lib Environment Variables

Think of `local::lib` like Python `venv`: it gives you a user-scoped install location so your modules and scripts do not touch the system Perl. Unlike `venv`, it does not create a separate Perl executable; it adjusts environment variables so Perl loads modules from your local tree first. These are typical environment variables produced by `local::lib` to install CPAN modules into a user directory instead of system-wide. 

PATH="/home/iwiseman/perl5/bin${PATH:+:${PATH}}"; export PATH;
PERL5LIB="/home/iwiseman/perl5/lib/perl5${PERL5LIB:+:${PERL5LIB}}"; export PERL5LIB;
PERL_LOCAL_LIB_ROOT="/home/iwiseman/perl5${PERL_LOCAL_LIB_ROOT:+:${PERL_LOCAL_LIB_ROOT}}"; export PERL_LOCAL_LIB_ROOT;
PERL_MB_OPT="--install_base \"/home/iwiseman/perl5\""; export PERL_MB_OPT;
PERL_MM_OPT="INSTALL_BASE=/home/iwiseman/perl5"; export PERL_MM_OPT;
  • PATH: prepends the local Perl `bin` so user-installed scripts are found first.
  • PERL5LIB: adds the local Perl library path so `use` and `require` can find modules.
  • PERL_LOCAL_LIB_ROOT: tracks the base directory used by `local::lib` (can be stacked).
  • PERL_MB_OPT: tells `Module::Build` to install into the local base.
  • PERL_MM_OPT: tells `ExtUtils::MakeMaker` to install into the local base.

Running

If you have Perl installed, you can run a script directly. 

perl script.pl

Debugging

Install the "Perl" extension for VS Code by Gerald Richter to get a Perl language server (syntax checking, symbol navigation, etc.) and debugger integration. VS Code can then debug with the Perl debugger using a launch configuration like this: 

{
  "version": "0.2.0",
  "configurations": [
    {
      "type": "perl",
      "request": "launch",
      "name": "Perl Debug App",
      "program": "${workspaceFolder}/script/app.pl",
      "args": ["daemon", "-l", "http://0.0.0.0:5001", "-m", "production"],
      "env": {
        "MOJO_REACTOR": "POLL"
      },
      "cwd": "${workspaceFolder}",
      "stopOnEntry": false,
      "reloadModules": true
    }
  ]
}

Package Management

Which package manager?

Perl modules can come from your OS or from CPAN.

  • OS packages (e.g. `apt install libmojolicious-perl`): stable, integrated with the distro; best for tools your whole system depends on.
  • CPAN + `cpan` client: ships with Perl; interactive shell for installing modules from CPAN.
  • CPAN + `cpanm` (cpanminus): lightweight command-line client; common choice for application projects, especially with `local::lib` or Carton.

For project code, prefer `cpanm` + a `cpanfile` over installing into the system Perl.

Listing what you have

Some ways to see what is installed or where it came from: 

# Show where a module is loaded from
perl -MData::Dumper -e 'print $INC{"Data/Dumper.pm"}, "\n";'

# Per-module location via perldoc
perldoc -l Mojolicious

# Distro-level Perl modules (Debian/Ubuntu)
apt list --installed '*perl*'

Installing modules

Using `cpanm` (recommended for app code): 

# Install a module into your local::lib tree
cpanm Mojolicious

# Install all dependencies for a project described in cpanfile
cpanm --installdeps .

Using the core `cpan` client: 

cpan Mojolicious

Using the OS package manager (Debian/Ubuntu example): 

sudo apt install libmojolicious-perl

Removing modules

Perl does not have a universal uninstall command for all clients, so strategy depends on how you installed:

  • OS packages: use the package manager (e.g. `sudo apt remove libmojolicious-perl`).
  • `local::lib` tree: you can safely remove or recreate the whole `/home/iwiseman/perl5` tree if it only holds project-local modules.
  • `cpanm`: recent versions support uninstall for some cases, but many people simply clean and reinstall into a fresh `local::lib`.

Searching for modules

Most searching is done via websites, but you can also search from the shell. 

# Search on MetaCPAN (browser)
https://metacpan.org

# Search in the legacy CPAN shell
cpan
cpan[1]> m /Mojo/

Avoiding and resolving conflicts

  • Avoid mixing system packages and CPAN installs for the same module (choose one source per module).
  • Use `local::lib` (and optionally Carton + `cpanfile`) so each project has its own module versions.
  • If things get messy in your local tree, it is often easier to remove `/home/iwiseman/perl5` and reinstall the modules you actually need.

Useful sites

Types

Perl has scalars, arrays, and hashes. Scalars can hold strings, numbers, and references.

  • Scalars: numbers, strings, references
  • Arrays: ordered lists
  • Hashes: key/value maps

Numbers

Perl uses numeric context for arithmetic. 

my $one = 1;          # integer
my $pi = 3.14159;     # float
my $big = 1_000_000;  # underscore for readability

Strings

Double quotes interpolate variables and escapes; single quotes do not. 

my $name = "Perl";
my $greet = "Hello, $name\n";
my $raw = 'Hello, $name\n';

print $greet;
print $raw;

Arrays

Arrays are ordered lists. 

my @numbers = (1, 2, 3, 4, 5);
print $numbers[2];  # 3

Hashes

Hashes map keys to values. 

my %ages = (
  Alice => 30,
  Bob   => 25,
);

print $ages{Alice};

Collections

Filtering and Mapping

Perl uses grep for filtering and map for transforms. 

my @ints = (1, 2, 3, 4, 5);

my @small = grep { $_ < 4 } @ints;
my @squares = map { $_ * $_ } @ints;

Combining

my @ints = (1, 2, 3, 4, 5);

my @small_squares = map { $_ * $_ }
                    grep { $_ < 4 } @ints;

Flow Control

If

if ($answer eq $correct_answer) {
  print "You were correct\n";
} else {
  print "Try again\n";
}

Given/When

Perl supports a switch-like feature via given/when (use with feature). Note: it is experimental in some versions. 

use feature 'switch';

my $number = 3;

given ($number) {
  when (0)    { print "Invalid number\n"; }
  when ([1,2]) { print "Number too low\n"; }
  when (3)    { print "Number correct\n"; }
  when (4)    { print "Number too high, but acceptable\n"; }
  default     { print "Number too high\n"; }
}

For and Foreach

for (my $i = 0; $i < 5; $i++) {
  print $i;
}

foreach my $n (@numbers) {
  print $n;
}

Community Notes

Popular Today

  • Perl 5 – the mainline Perl most people mean when they say “Perl”; actively maintained, widely deployed in sysadmin, web, data-processing and scripting.
  • Mojolicious – modern, non-blocking web framework (routers, templates, testing helpers, WebSockets, etc.); good default choice for new web apps.
  • DBI / DBD::* – standard way to talk to databases (e.g. `DBD::Pg` for Postgres); forms the base for higher-level ORMs.
  • Carton / cpanminus – common tools to manage project-specific dependencies from CPAN (`cpanm`) and lockfiles (`Carton`) without touching system Perl.

Less Popular / Legacy

  • Perl 6 / Raku – Perl 6 was renamed to Raku and is now its own language and community; interesting, but not a path for “upgrading” Perl 5 code.
  • CGI.pm – classic module for raw CGI-style web programming; still works, but most new code uses PSGI/Plack or frameworks like Mojolicious instead.
  • mod_perl-only stacks – historically popular for embedding Perl in Apache; many new projects prefer FastCGI, PSGI/Plack, or Mojolicious daemons behind nginx/Apache.

Starting a Real Project: dvdrental_api_pl

This section sketches how to go from an empty directory to the current dvdrental_api_pl REST API setup. Details of routes and database schema can live on a separate page.

1. Create the project skeleton

mkdir -p dvdrental_api_pl/{lib,script}
cd dvdrental_api_pl

# Optional: initialise git
git init

Recommended layout for this project:

  • script/app.pl – Mojolicious application entry point (matches the VS Code debug config).
  • lib/DVD/... – your application modules (e.g. DVD::API, DVD::Model::Rental).
  • Documentation/ – wiki pages like this one.

2. Set up local Perl environment

Use local::lib (or similar) so project modules do not touch the system Perl. 

perl -Mlocal::lib >> ~/.bashrc
source ~/.bashrc

After that, verify the environment variables look like the ones described in the local::lib section above (PATH, PERL5LIB, etc.).

3. Install project dependencies

Use cpanm with a cpanfile (recommended). A minimal cpanfile for a Mojolicious + Postgres REST API might look like: 

requires 'Mojolicious';
requires 'DBI';
requires 'DBD::Pg';
requires 'JSON::MaybeXS';

Then install: 

cpanm --installdeps .

You can also mix in OS packages for heavy C libraries (e.g. libpq-dev) as shown in the Package Installation section.

4. Create a minimal Mojolicious REST stub

Create script/app.pl to expose at least one test route. 

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

use Mojolicious::Lite;

get '/health' => sub ($c) {
  $c->render(json => { status => 'ok' });
};

app->start;

Run it manually to confirm it works: 

perl script/app.pl daemon -l http://0.0.0.0:5001

curl http://localhost:5001/health

Later, this stub can be replaced by a full application class living under lib/DVD/..., but this is enough to validate the toolchain.

5. Hook up VS Code debugging

Use the launch configuration shown earlier ("Perl Debug App") so VS Code runs script/app.pl in daemon mode:

  • Program: ${workspaceFolder}/script/app.pl
  • Args: ["daemon", "-l", "http://0.0.0.0:5001", "-m", "production"]
  • Env: MOJO_REACTOR=POLL

Set a breakpoint inside the /health route and verify that requests from curl or a browser hit the breakpoint.

6. Connect to the dvdrental database

Once the basic REST skeleton works, add a module under lib/DVD/ that uses DBI/DBD::Pg to talk to the dvdrental Postgres database. A separate page can describe:

  • Connection DSN and credentials.
  • Simple SELECT examples.
  • How routes map to SQL queries.

At this point you have:

  • A local Perl environment (local::lib).
  • Dependencies managed via cpanm/cpanfile.
  • A Mojolicious app entry script with debug support in VS Code.
  • A place (lib/DVD/...) to grow the dvdrental_api_pl REST API.
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