Perl references explained

Coming to Perl from PHP can be confusing because of the apparently similar and yet quite different ways the two languages use the $ identifier as a variable prefix. If you’re accustomed to PHP, you’ll be used to declaring variables of various types, using the $ prefix each time:

<?php
$string = "string";
$integer = 6;
$float = 1.337;
$object = new Object();

So when you begin working in Perl, you’re lulled into a false sense of security because it looks like you can do the same thing with any kind of data:

#!/usr/bin/perl
my $string = "string";
my $integer = 6;
my $float = 1.337;
my $object = Object->new();

But then you start dealing with arrays and hashes, and suddenly everything stops working the way you expect. Consider this snippet:

#!/usr/bin/perl
my $array = (1, 2, 3);
print $array. "\n";

That’s perfectly valid syntax. However, when you run it, expecting to see all your elements or at least an Array like in PHP, you get the output of “3”. Not being able to assign a list to a scalar, Perl just gives you the last item of the list instead. You sit there confused, wondering how on earth Perl could think that’s what you meant.

References in PHP

In PHP, every identifier is a reference, or pointer, towards some underlying data. PHP handles the memory management for you. When you declare an object, PHP writes the data into memory, and puts into the variable you define a reference to that data. The variable is not the data itself, it’s just a pointer to it. To oversimplify things a bit, what’s actually stored in your $variable is an address in memory, and not a sequence of data.

When PHP manages all this for you and you’re writing basic programs, you don’t really notice, because any time you actually use the value it gets dereferenced implicitly, meaning that PHP will use the data that the variable points to. So when you write something like:

$string = "string";
print $string;

The output you get is “string”, and not “0x00abf0a9″, which might be the “real” value of $string as an address in memory. In this way, PHP is kind of coddling you a bit. In fact, if you actually want two identifiers to point to the same piece of data rather than making a copy in memory, you have to use a special reference syntax:

$string2 = &$string1;

Perl and C programmers aren’t quite as timid about hiding references, because being able to manipulate them a bit more directly turns out to be very useful for writing quick, clean code, in particular for conserving memory and dealing with state intelligently.

References in Perl

In Perl, you have three basic data types; scalars, arrays, and hashes. These all use different identifiers; Scalars use $, arrays use @, and hashes use %.

my $string = "string";
my $integer = 0;
my $float = 0.0;
my @array = (1,2,3);
my %hash = (name => "Tom Ryder",
            blog => "Arabesque");

So scalars can refer directly to data in the way you’re accustomed to in PHP, but they can also be references to any other kind of data. For example, you could write:

my $string = "string";
my $copy = $string;
my $reference = \$string;

The value of both $string and $copy, when printed, will be “string”, as you might expect. However, the $reference scalar becomes a reference to the data stored in $string, and when printed out would give something like SCALAR(0x2160718). Similarly, you can define a scalar as a reference to an array or hash:

my @array = (1,2,3);
my $arrayref = \@array;
my %hash = (name => "Tom Ryder",
            blog => "Arabesque");
my $hashref = \%hash;

There are even shorthands for doing this, if you want to declare a reference and the data it references inline. For array references, you use square brackets, and for hash references, curly brackets:

$arrayref = [1,2,3];
$hashref = {name => "Tom Ryder",
            blog => "Arabesque"};

And if you really do want to operate with the data of the reference, rather than the reference itself, you can explicitly dereference it:

$string = ${$reference};
@array = @{$arrayref};
%hash = %{$hashref};

For a much more in-depth discussion of how references work in Perl and their general usefulness, check out perlref in the Perl documentation.