# Perl Quick Reference

*Variables, regex, file I/O, references, modules essentials*

> Source: Perl Documentation (perldoc.perl.org) · MIT

## Basics

### Hello World

```
#!/usr/bin/perl
use strict;
use warnings;
print "Hello, World!\n";
say "Hello, World!";  # with use feature 'say';
```

### Run Perl

```
perl script.pl           # run a file
perl -e 'print "hi\n"'  # run inline
perl -ne 'print' file   # process file line by line
```

### Comments & Documentation

```
# single-line comment
=pod
Multi-line POD documentation
=cut
```

## Variables

### Sigils

| Command | Description |
|---------|-------------|
| `$scalar` | Single value (string, number, reference) |
| `@array` | Ordered list of scalars |
| `%hash` | Key-value pairs |
| `$array[0]` | Access single array element (scalar context) |
| `$hash{key}` | Access single hash value (scalar context) |

### Scalar Variables

```
my $name = "Perl";        # string
my $version = 5.40;       # number
my $count = 42;           # integer
my $undef;                # undefined (undef)
my $combined = "$name v$version";  # interpolation
```

### Context

```
my @arr = (1, 2, 3);
my $count = @arr;          # scalar context: 3
my @copy = @arr;           # list context: (1, 2, 3)
my $len = scalar @arr;     # force scalar context
```

### Special Variables

| Command | Description |
|---------|-------------|
| `$_` | Default variable (topic) |
| `@_` | Subroutine arguments |
| `$!` | System error message |
| `$@` | Error from eval |
| `$0` | Program name |
| `@ARGV` | Command-line arguments |
| `%ENV` | Environment variables |

## Operators

### Comparison Operators

| Command | Description |
|---------|-------------|
| `==, !=, <, >, <=, >=` | Numeric comparison |
| `eq, ne, lt, gt, le, ge` | String comparison |
| `<=>` | Numeric spaceship (returns -1, 0, 1) |
| `cmp` | String spaceship |
| `=~` | Regex match / bind |
| `!~` | Regex negated match |

### String Operators

```
my $full = "Hello" . " " . "World";  # concatenation
my $line = "-" x 40;                  # repetition
my $len = length($full);              # 11
```

### Logical Operators

| Command | Description |
|---------|-------------|
| `&& / and` | Logical AND (low precedence: `and`) |
| `\|\| / or` | Logical OR (low precedence: `or`) |
| `// ` | Defined-or (returns left if defined) |
| `! / not` | Logical NOT |
| `? :` | Ternary conditional |

## Control Flow

### Conditionals

```
if ($x > 0) { print "positive\n"; }
elsif ($x == 0) { print "zero\n"; }
else { print "negative\n"; }
print "yes\n" if $condition;     # postfix if
print "no\n" unless $condition;  # postfix unless
```

### Loops

```
for my $i (0..9) { print "$i\n"; }
foreach my $item (@array) { print "$item\n"; }
while ($line = <STDIN>) { chomp $line; }
until ($done) { last if check(); }
```

### Loop Control

| Command | Description |
|---------|-------------|
| `next` | Skip to next iteration (like `continue`) |
| `last` | Exit loop (like `break`) |
| `redo` | Restart current iteration |
| `next LABEL` | Skip to next iteration of labeled loop |
| `last LABEL` | Exit labeled loop |

### Given / When

```
use feature 'switch';
given ($status) {
    when ("ok")    { say "success"; }
    when ("error") { say "failed"; }
    default        { say "unknown"; }
}
```

## Subroutines

### Basic Subroutine

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

### Default & Named Parameters

```
sub connect {
    my (%opts) = @_;
    my $host = $opts{host} // "localhost";
    my $port = $opts{port} // 5432;
    return "$host:$port";
}
connect(host => "db.example.com", port => 3306);
```

### Subroutine References

```
my $double = sub { return $_[0] * 2; };
print $double->(5);   # 10
my @sorted = sort { $a <=> $b } @nums;
```

### Prototypes & Signatures

```
use feature 'signatures';
sub add($a, $b) { return $a + $b; }
sub greet($name, $greeting = "Hello") {
    return "$greeting, $name!";
}
```

## Regex

### Matching

```
if ($str =~ /pattern/) { print "matched\n"; }
if ($str =~ /(\d+)/) { print "number: $1\n"; }
my @matches = ($str =~ /(\w+)/g);  # all matches
```

### Substitution

```
$str =~ s/old/new/;      # first occurrence
$str =~ s/old/new/g;     # global (all occurrences)
$str =~ s/^\s+|\s+$//g;  # trim whitespace
(my $clean = $str) =~ s/\W//g;  # non-destructive copy
```

### Modifiers

| Command | Description |
|---------|-------------|
| `/i` | Case-insensitive |
| `/g` | Global (all matches) |
| `/m` | Multi-line (`^` and `$` match line boundaries) |
| `/s` | Single-line (`.` matches newline) |
| `/x` | Extended (allow whitespace and comments) |

### Common Patterns

| Command | Description |
|---------|-------------|
| `\d, \D` | Digit / non-digit |
| `\w, \W` | Word char / non-word char |
| `\s, \S` | Whitespace / non-whitespace |
| `\b` | Word boundary |
| `(?: ... )` | Non-capturing group |
| `(?<name> ... )` | Named capture (access via `$+{name}`) |

## File I/O

### Open & Read

```
open(my $fh, '<', 'data.txt') or die "Cannot open: $!";
while (my $line = <$fh>) {
    chomp $line;
    print "$line\n";
}
close($fh);
```

### Write & Append

```
open(my $fh, '>', 'out.txt') or die "Cannot open: $!";
print $fh "Hello\n";
close($fh);
open(my $fh, '>>', 'log.txt') or die "Cannot open: $!";
print $fh "entry\n";
close($fh);
```

### Slurp Entire File

```
use File::Slurp;
my $content = read_file('data.txt');
my @lines = read_file('data.txt', chomp => 1);
```

### File Tests

| Command | Description |
|---------|-------------|
| `-e $path` | File exists |
| `-f $path` | Is a regular file |
| `-d $path` | Is a directory |
| `-r / -w / -x` | Readable / writable / executable |
| `-s $path` | File size in bytes (0 if empty) |
| `-z $path` | File is zero size |

## Arrays & Hashes

### Arrays

```
my @arr = (1, 2, 3, 4, 5);
push @arr, 6;              # append
my $last = pop @arr;       # remove last
my $first = shift @arr;    # remove first
unshift @arr, 0;           # prepend
my @slice = @arr[1..3];    # slice
```

### Array Functions

| Command | Description |
|---------|-------------|
| `scalar @arr` | Number of elements |
| `push / pop` | Add/remove from end |
| `shift / unshift` | Remove/add from beginning |
| `splice(@a, 2, 1)` | Remove 1 element at index 2 |
| `sort @arr` | Sort alphabetically |
| `reverse @arr` | Reverse order |
| `grep { /pat/ } @arr` | Filter by pattern |
| `map { $_ * 2 } @arr` | Transform each element |
| `join(',', @arr)` | Join into string |

### Hashes

```
my %user = (name => "Alice", age => 30);
$user{email} = "a\@b.com";    # add pair
delete $user{age};             # remove pair
my @keys = keys %user;
my @vals = values %user;
```

### Hash Iteration

```
while (my ($k, $v) = each %hash) {
    print "$k => $v\n";
}
for my $key (sort keys %hash) {
    print "$key: $hash{$key}\n";
}
```

## References

### Creating References

```
my $scalar_ref = \$name;
my $array_ref  = \@arr;
my $hash_ref   = \%hash;
my $anon_arr   = [1, 2, 3];       # anonymous array ref
my $anon_hash  = {a => 1, b => 2}; # anonymous hash ref
```

### Dereferencing

```
print $$scalar_ref;          # dereference scalar
print $array_ref->[0];       # arrow notation
print $hash_ref->{key};      # arrow notation
my @copy = @$array_ref;      # dereference to array
my %copy = %$hash_ref;       # dereference to hash
```

### Complex Data Structures

```
my @users = (
    { name => "Alice", age => 30 },
    { name => "Bob",   age => 25 },
);
print $users[0]->{name};   # "Alice"
```

### ref() Function

| Command | Description |
|---------|-------------|
| `ref($r) eq 'SCALAR'` | Reference to scalar |
| `ref($r) eq 'ARRAY'` | Reference to array |
| `ref($r) eq 'HASH'` | Reference to hash |
| `ref($r) eq 'CODE'` | Reference to subroutine |

## Modules

### Using Modules

```
use strict;
use warnings;
use List::Util qw(sum max min);
use File::Basename;
use Cwd qw(abs_path);
```

### Creating a Module

```
# MyModule.pm
package MyModule;
use Exporter 'import';
our @EXPORT_OK = qw(helper);
sub helper { return "help"; }
1;  # module must return true
```

### Common Core Modules

| Command | Description |
|---------|-------------|
| `List::Util` | sum, max, min, reduce, any, all |
| `File::Basename` | basename, dirname, fileparse |
| `File::Path` | make_path, remove_tree |
| `Getopt::Long` | Command-line option parsing |
| `JSON` | encode_json, decode_json |
| `LWP::Simple` | get($url) — simple HTTP client |
| `Data::Dumper` | Debug dump of data structures |
| `Carp` | croak, confess — better error messages |

### CPAN

```
cpan install Module::Name        # install from CPAN
cpanm Module::Name               # cpanminus (faster)
perldoc Module::Name             # read module docs
```