Perl Patterns

---
name: perl-patterns
description: Modern Perl 5.36+ idioms, best practices, and conventions for building robust, maintainable Perl applications. Use when writing or reviewing modern Perl 5.36+ code.
metadata:
  origin: ECC
---

# Modern Perl Development Patterns

Idiomatic Perl 5.36+ patterns and best practices for building robust, maintainable applications.

## When to Activate

- Writing new Perl code or modules
- Reviewing Perl code for idiom compliance
- Refactoring legacy Perl to modern standards
- Designing Perl module architecture
- Migrating pre-5.36 code to modern Perl

## How It Works

Apply these patterns as a bias toward modern Perl 5.36+ defaults: signatures, explicit modules, focused error handling, and testable boundaries. The examples below are meant to be copied as starting points, then tightened for the actual app, dependency stack, and deployment model in front of you.

## Core Principles

### 1. Use `v5.36` Pragma

A single `use v5.36` replaces the old boilerplate and enables strict, warnings, and subroutine signatures.

```perl
# Good: Modern preamble
use v5.36;

sub greet($name) {
    say "Hello, $name!";
}

# Bad: Legacy boilerplate
use strict;
use warnings;
use feature 'say', 'signatures';
no warnings 'experimental::signatures';

sub greet {
    my ($name) = @_;
    say "Hello, $name!";
}
```

### 2. Subroutine Signatures

Use signatures for clarity and automatic arity checking.

```perl
use v5.36;

# Good: Signatures with defaults
sub connect_db($host, $port = 5432, $timeout = 30) {
    # $host is required, others have defaults
    return DBI->connect("dbi:Pg:host=$host;port=$port", undef, undef, {
        RaiseError => 1,
        PrintError => 0,
    });
}

# Good: Slurpy parameter for variable args
sub log_message($level, @details) {
    say "[$level] " . join(' ', @details);
}

# Bad: Manual argument unpacking
sub connect_db {
    my ($host, $port, $timeout) = @_;
    $port    //= 5432;
    $timeout //= 30;
    # ...
}
```

### 3. Context Sensitivity

Understand scalar vs list context — a core Perl concept.

```perl
use v5.36;

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

my @copy  = @items;            # List context: all elements
my $count = @items;            # Scalar context: count (5)
say "Items: " . scalar @items; # Force scalar context
```

### 4. Postfix Dereferencing

Use postfix dereference syntax for readability with nested structures.

```perl
use v5.36;

my $data = {
    users => [
        { name => 'Alice', roles => ['admin', 'user'] },
        { name => 'Bob',   roles => ['user'] },
    ],
};

# Good: Postfix dereferencing
my @users = $data->{users}->@*;
my @roles = $data->{users}[0]{roles}->@*;
my %first = $data->{users}[0]->%*;

# Bad: Circumfix dereferencing (harder to read in chains)
my @users = @{ $data->{users} };
my @roles = @{ $data->{users}[0]{roles} };
```

### 5. The `isa` Operator (5.32+)

Infix type-check — replaces `blessed($o) && $o->isa('X')`.

```perl
use v5.36;
if ($obj isa 'My::Class') { $obj->do_something }
```

## Error Handling

### eval/die Pattern

```perl
use v5.36;

sub parse_config($path) {
    my $content = eval { path($path)->slurp_utf8 };
    die "Config error: $@" if $@;
    return decode_json($content);
}
```

### Try::Tiny (Reliable Exception Handling)

```perl
use v5.36;
use Try::Tiny;

sub fetch_user($id) {
    my $user = try {
        $db->resultset('User')->find($id)
            // die "User $id not found\n";
    }
    catch {
        warn "Failed to fetch user $id: $_";
        undef;
    };
    return $user;
}
```

### Native try/catch (5.40+)

```perl
use v5.40;

sub divide($x, $y) {
    try {
        die "Division by zero" if $y == 0;
        return $x / $y;
    }
    catch ($e) {
        warn "Error: $e";
        return;
    }
}
```

## Modern OO with Moo

Prefer Moo for lightweight, modern OO. Use Moose only when its metaprotocol is needed.

```perl
# Good: Moo class
package User;
use Moo;
use Types::Standard qw(Str Int ArrayRef);
use namespace::autoclean;

has name  => (is => 'ro', isa => Str, required => 1);
has email => (is => 'ro', isa => Str, required => 1);
has age   => (is => 'ro', isa => Int, default  => sub { 0 });
has roles => (is => 'ro', isa => ArrayRef[Str], default => sub { [] });

sub is_admin($self) {
    return grep { $_ eq 'admin' } $self->roles->@*;
}

sub greet($self) {
    return "Hello, I'm " . $self->name;
}

1;

# Usage
my $user = User->new(
    name  => 'Alice',
    email => 'alice@example.com',
    roles => ['admin', 'user'],
);

# Bad: Blessed hashref (no validation, no accessors)
package User;
sub new {
    my ($class, %args) = @_;
    return bless \%args, $class;
}
sub name { return $_[0]->{name} }
1;
```

### Moo Roles

```perl
package Role::Serializable;
use Moo::Role;
use JSON::MaybeXS qw(encode_json);
requires 'TO_HASH';
sub to_json($self) { encode_json($self->TO_HASH) }
1;

package User;
use Moo;
with 'Role::Serializable';
has name  => (is => 'ro', required => 1);
has email => (is => 'ro', required => 1);
sub TO_HASH($self) { { name => $self->name, email => $self->email } }
1;
```

### Native `class` Keyword (5.38+, Corinna)

```perl
use v5.38;
use feature 'class';
no warnings 'experimental::class';

class Point {
    field $x :param;
    field $y :param;
    method magnitude() { sqrt($x**2 + $y**2) }
}

my $p = Point->new(x => 3, y => 4);
say $p->magnitude;  # 5
```

## Regular Expressions

### Named Captures and `/x` Flag

```perl
use v5.36;

# Good: Named captures with /x for readability
my $log_re = qr{
    ^ (?<timestamp> \d{4}-\d{2}-\d{2} \s \d{2}:\d{2}:\d{2} )
    \s+ \[ (?<level> \w+ ) \]
    \s+ (?<message> .+ ) $
}x;

if ($line =~ $log_re) {
    say "Time: $+{timestamp}, Level: $+{level}";
    say "Message: $+{message}";
}

# Bad: Positional captures (hard to maintain)
if ($line =~ /^(\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2})\s+\[(\w+)\]\s+(.+)$/) {
    say "Time: $1, Level: $2";
}
```

### Precompiled Patterns

```perl
use v5.36;

# Good: Compile once, use many
my $email_re = qr/^[A-Za-z0-9._%+-]+\@[A-Za-z0-9.-]+\.[A-Za-z]{2,}$/;

sub validate_emails(@emails) {
    return grep { $_ =~ $email_re } @emails;
}
```

## Data Structures

### References and Safe Deep Access

```perl
use v5.36;

# Hash and array references
my $config = {
    database => {
        host => 'localhost',
        port => 5432,
        options => ['utf8', 'sslmode=require'],
    },
};

# Safe deep access (returns undef if any level missing)
my $port = $config->{database}{port};           # 5432
my $missing = $config->{cache}{host};           # undef, no error

# Hash slices
my %subset;
@subset{qw(host port)} = @{$config->{database}}{qw(host port)};

# Array slices
my @first_two = $config->{database}{options}->@[0, 1];

# Multi-variable for loop (experimental in 5.36, stable in 5.40)
use feature 'for_list';
no warnings 'experimental::for_list';
for my ($key, $val) (%$config) {
    say "$key => $val";
}
```

## File I/O

### Three-Argument Open

```perl
use v5.36;

# Good: Three-arg open with autodie (core module, eliminates 'or die')
use autodie;

sub read_file($path) {
    open my $fh, '<:encoding(UTF-8)', $path;
    local $/;
    my $content = <$fh>;
    close $fh;
    return $content;
}

# Bad: Two-arg open (shell injection risk, see perl-security)
open FH, $path;            # NEVER do this
open FH, "< $path";        # Still bad — user data in mode string
```

### Path::Tiny for File Operations

```perl
use v5.36;
use Path::Tiny;

my $file = path('config', 'app.json');
my $content = $file->slurp_utf8;
$file->spew_utf8($new_content);

# Iterate directory
for my $child (path('src')->children(qr/\.pl$/)) {
    say $child->basename;
}
```

## Module Organization

### Standard Project Layout

```text
MyApp/
├── lib/
│   └── MyApp/
│       ├── App.pm           # Main module
│       ├── Config.pm        # Configuration
│       ├── DB.pm            # Database layer
│       └── Util.pm          # Utilities
├── bin/
│   └── myapp                # Entry-point script
├── t/
│   ├── 00-load.t            # Compilation tests
│   ├── unit/                # Unit tests
│   └── integration/         # Integration tests
├── cpanfile                 # Dependencies
├── Makefile.PL              # Build system
└── .perlcriticrc            # Linting config
```

### Exporter Patterns

```perl
package MyApp::Util;
use v5.36;
use Exporter 'import';

our @EXPORT_OK   = qw(trim);
our %EXPORT_TAGS = (all => \@EXPORT_OK);

sub trim($str) { $str =~ s/^\s+|\s+$//gr }

1;
```

## Tooling

### perltidy Configuration (.perltidyrc)

```text
-i=4        # 4-space indent
-l=100      # 100-char line length
-ci=4       # continuation indent
-ce         # cuddled else
-bar        # opening brace on same line
-nolq       # don't outdent long quoted strings
```

### perlcritic Configuration (.perlcriticrc)

```ini
severity = 3
theme = core + pbp + security

[InputOutput::RequireCheckedSyscalls]
functions = :builtins
exclude_functions = say print

[Subroutines::ProhibitExplicitReturnUndef]
severity = 4

[ValuesAndExpressions::ProhibitMagicNumbers]
allowed_values = 0 1 2 -1
```

### Dependency Management (cpanfile + carton)

```bash
cpanm App::cpanminus Carton   # Install tools
carton install                 # Install deps from cpanfile
carton exec -- perl bin/myapp  # Run with local deps
```

```perl
# cpanfile
requires 'Moo', '>= 2.005';
requires 'Path::Tiny';
requires 'JSON::MaybeXS';
requires 'Try::Tiny';

on test => sub {
    requires 'Test2::V0';
    requires 'Test::MockModule';
};
```

## Quick Reference: Modern Perl Idioms

| Legacy Pattern | Modern Replacement |
|---|---|
| `use strict; use warnings;` | `use v5.36;` |
| `my ($x, $y) = @_;` | `sub foo($x, $y) { ... }` |
| `@{ $ref }` | `$ref->@*` |
| `%{ $ref }` | `$ref->%*` |
| `open FH, "< $file"` | `open my $fh, '<:encoding(UTF-8)', $file` |
| `blessed hashref` | `Moo` class with types |
| `$1, $2, $3` | `$+{name}` (named captures) |
| `eval { }; if ($@)` | `Try::Tiny` or native `try/catch` (5.40+) |
| `BEGIN { require Exporter; }` | `use Exporter 'import';` |
| Manual file ops | `Path::Tiny` |
| `blessed($o) && $o->isa('X')` | `$o isa 'X'` (5.32+) |
| `builtin::true / false` | `use builtin 'true', 'false';` (5.36+, experimental) |

## Anti-Patterns

```perl
# 1. Two-arg open (security risk)
open FH, $filename;                     # NEVER

# 2. Indirect object syntax (ambiguous parsing)
my $obj = new Foo(bar => 1);            # Bad
my $obj = Foo->new(bar => 1);           # Good

# 3. Excessive reliance on $_
map { process($_) } grep { validate($_) } @items;  # Hard to follow
my @valid = grep { validate($_) } @items;           # Better: break it up
my @results = map { process($_) } @valid;

# 4. Disabling strict refs
no strict 'refs';                        # Almost always wrong
${"My::Package::$var"} = $value;         # Use a hash instead

# 5. Global variables as configuration
our $TIMEOUT = 30;                       # Bad: mutable global
use constant TIMEOUT => 30;              # Better: constant
# Best: Moo attribute with default

# 6. String eval for module loading
eval "require $module";                  # Bad: code injection risk
eval "use $module";                      # Bad
use Module::Runtime 'require_module';    # Good: safe module loading
require_module($module);
```

**Remember**: Modern Perl is clean, readable, and safe. Let `use v5.36` handle the boilerplate, use Moo for objects, and prefer CPAN's battle-tested modules over hand-rolled solutions.