Perl Patterns

# Good: Modern preambleuse v5.36;
sub greet($name) {    say "Hello, $name!";}
# Bad: Legacy boilerplateuse strict;use warnings;use feature 'say', 'signatures';no warnings 'experimental::signatures';
sub greet {    my ($name) = @_;    say "Hello, $name!";}

use v5.36;
# Good: Signatures with defaultssub connect_db($host, $port = 5432, $timeout = 30) {    # $host is required, others have defaults    return DBI->connect("dbi:Pg:host=$host;port=$port", undef, undef, {        RaiseError => 1,        PrintError => 0,    });}
# Good: Slurpy parameter for variable argssub log_message($level, @details) {    say "[$level] " . join(' ', @details);}
# Bad: Manual argument unpackingsub connect_db {    my ($host, $port, $timeout) = @_;    $port    //= 5432;    $timeout //= 30;    # ...}

use v5.36;
my @items = (1, 2, 3, 4, 5);
my @copy  = @items;            # List context: all elementsmy $count = @items;            # Scalar context: count (5)say "Items: " . scalar @items; # Force scalar context

use v5.36;
my $data = {    users => [        { name => 'Alice', roles => ['admin', 'user'] },        { name => 'Bob',   roles => ['user'] },    ],};
# Good: Postfix dereferencingmy @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} };

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

use v5.36;
sub parse_config($path) {    my $content = eval { path($path)->slurp_utf8 };    die "Config error: $@" if $@;    return decode_json($content);}

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;}

use v5.40;
sub divide($x, $y) {    try {        die "Division by zero" if $y == 0;        return $x / $y;    }    catch ($e) {        warn "Error: $e";        return;    }}

# Good: Moo classpackage 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;
# Usagemy $user = User->new(    name  => 'Alice',    email => '[email protected]',    roles => ['admin', 'user'],);
# Bad: Blessed hashref (no validation, no accessors)package User;sub new {    my ($class, %args) = @_;    return bless \%args, $class;}sub name { return $_[0]->{name} }1;

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;

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

use v5.36;
# Good: Named captures with /x for readabilitymy $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";}

use v5.36;
# Good: Compile once, use manymy $email_re = qr/^[A-Za-z0-9._%+-]+\@[A-Za-z0-9.-]+\.[A-Za-z]{2,}$/;
sub validate_emails(@emails) {    return grep { $_ =~ $email_re } @emails;}

use v5.36;
# Hash and array referencesmy $config = {    database => {        host => 'localhost',        port => 5432,        options => ['utf8', 'sslmode=require'],    },};
# Safe deep access (returns undef if any level missing)my $port = $config->{database}{port};           # 5432my $missing = $config->{cache}{host};           # undef, no error
# Hash slicesmy %subset;@subset{qw(host port)} = @{$config->{database}}{qw(host port)};
# Array slicesmy @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";}

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 thisopen FH, "< $path";        # Still bad — user data in mode string

use v5.36;use Path::Tiny;
my $file = path('config', 'app.json');my $content = $file->slurp_utf8;$file->spew_utf8($new_content);
# Iterate directoryfor my $child (path('src')->children(qr/\.pl$/)) {    say $child->basename;}

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;

# cpanfilerequires 'Moo', '>= 2.005';requires 'Path::Tiny';requires 'JSON::MaybeXS';requires 'Try::Tiny';
on test => sub {    requires 'Test2::V0';    requires 'Test::MockModule';};

# 1. Two-arg open (security risk)open FH, $filename;                     # NEVER
# 2. Indirect object syntax (ambiguous parsing)my $obj = new Foo(bar => 1);            # Badmy $obj = Foo->new(bar => 1);           # Good
# 3. Excessive reliance on $_map { process($_) } grep { validate($_) } @items;  # Hard to followmy @valid = grep { validate($_) } @items;           # Better: break it upmy @results = map { process($_) } @valid;
# 4. Disabling strict refsno strict 'refs';                        # Almost always wrong${"My::Package::$var"} = $value;         # Use a hash instead
# 5. Global variables as configurationour $TIMEOUT = 30;                       # Bad: mutable globaluse constant TIMEOUT => 30;              # Better: constant# Best: Moo attribute with default
# 6. String eval for module loadingeval "require $module";                  # Bad: code injection riskeval "use $module";                      # Baduse Module::Runtime 'require_module';    # Good: safe module loadingrequire_module($module);