References and Data Structures in Perl

Arrays and hashes only hold scalars — so to build anything nested, you need references. A reference is a scalar that points at another value. This is the key to real data structures in Perl.

Making and using references

Put a \ in front of a variable to get a reference to it:

my @colors = ("red", "green", "blue");
my $aref   = \@colors;   # reference to the array

my %ages = (Ada => 36);
my $href = \%ages;       # reference to the hash

Create anonymous references directly with [] and {}:

my $aref = ["red", "green", "blue"];   # anonymous array ref
my $href = { Ada => 36, Bo => 41 };    # anonymous hash ref

Dereferencing

To get back to the data, use the arrow -> for single elements:

print $aref->[0];      # red
print $href->{Ada};    # 36

To get the whole thing back:

my @all  = @$aref;   # or @{ $aref }
my %copy = %$href;

The arrow rule

Between two sets of brackets, the arrow is optional. These are identical:

$data->{users}[0]{name}
$data->{users}->[0]->{name}

Arrays of hashes

A common shape — a list of records:

my @people = (
    { name => "Ada", role => "eng" },
    { name => "Bo",  role => "sales" },
);

for my $p (@people) {
    print "$p->{name} works in $p->{role}\n";
}

Hashes of arrays

Group things under keys:

my %team = (
    eng   => ["Ada", "Cy"],
    sales => ["Bo"],
);

push @{ $team{eng} }, "Dev";      # add to the eng list
print "eng: @{ $team{eng} }\n";   # eng: Ada Cy Dev

Nesting arbitrarily

Because a reference is just a scalar, you can nest to any depth:

my $company = {
    name  => "Perl Archive",
    teams => {
        eng => [
            { name => "Ada", langs => ["perl", "c"] },
        ],
    },
};

print $company->{teams}{eng}[0]{langs}[0];   # perl

Inspecting a structure

When a nested structure confuses you, dump it:

use Data::Dumper;
print Dumper($company);

Where to go next

Return to the Perl Archive hub for the full set of guides, or start at the beginning with the Perl Tutorial for Beginners.