Answer.

Perl does not have a built-in syntax for multidimensional arrays like some other languages do. Instead, it uses arrays of arrays, where each top-level element is a reference to another array. This organization allows for flexible modeling of tables, matrices, and other data structures requiring two-dimensionality or greater nesting.

Background

Originally, Perl was developed for text processing and working with simple structures, but with the introduction of references (starting with Perl 5), developers gained the ability to build complex nested structures — for example, an array of arrays or a hash of arrays.

Problem

The main misconception for new users is the attempt to create a two-dimensional array simply — for example, declaring @matrix = ( (1,2), (3,4) ). This approach will not yield the desired result, as elements will be unpacked as scalar values rather than as nested structures. A common mistake when copying arrays is shallow copying, which can lead to unexpected side effects.

Solution

In Perl, multidimensional arrays are built through references to arrays. Correct initialization looks like this:

my @matrix;
for my $i (0..2) {
    for my $j (0..2) {
        $matrix[$i][$j] = $i * $j;
    }
}
# Accessing an element: $matrix[1][2]

Or through anonymous references:

my $matrix = [ [1,2,3], [4,5,6], [7,8,9] ];
print $matrix->[1][2]; # 6

Key features:

All nested structures are references: modifying a nested array may impact other parts of the data if copied incorrectly
There is no syntactic sugar for creating and initializing multidimensional arrays; everything is done explicitly
Deep copying is required for copying a multidimensional array; otherwise, you risk getting shared memory sections

Trick Questions.

Is it possible to create multidimensional arrays without references, just declaring parentheses inside parentheses as in other languages?

No. In this case, Perl dereferences elements as a regular list. Only using references is correct.

Example of incorrect code:

my @matrix = ((1,2,3),(4,5,6),(7,8,9)); # Elements are flattened
print $matrix[3]; # 4, not [4,5,6] — incorrect behavior

Correct way:

my @matrix = ( [1,2,3], [4,5,6], [7,8,9] );
print $matrix[1][2]; # 6

What happens if you copy an array of arrays with simple assignment?

Only the top level is copied; nested arrays will refer to the same memory locations.

Example:

my @a = ( [1,2], [3,4] );
my @b = @a;
$a[0][0] = 99;
print $b[0][0]; # 99, although we expected 1 — shallow copy!

Can a nested array be "deeply" copied with Perl's built-in capabilities?

No, Perl does not provide a standard deep copy operator for nested structures. You need to use the Storable module or a recursive function.

Example with Storable:

use Storable 'dclone';
my $deepcopy = dclone(\@matrix);

Common mistakes and anti-patterns

Attempting to copy a multidimensional array with simple assignment
Lack of deep copying when working with nested structures
Attempting to access uninitialized elements, which leads to errors (autovivification)
Mixing scalars and references in one structure

Real-life examples

Negative case

A developer creates a two-dimensional matrix with a simple array declaration and copies it with assignment:

my @m1 = ([1,2],[3,4]);
my @m2 = @m1;
$m1[0][0] = 77;
print $m2[0][0];

Advantages:

Simple and fast
Code is easily readable by a novice

Disadvantages:

Structure changes unexpectedly in both arrays
Potential bugs in a large project

Positive case

Using the Storable module for deep copying:

use Storable 'dclone';
my @m1 = ([1,2],[3,4]);
my $m2 = dclone(\@m1);
$m1[0][0] = 77;
print $m2->[0][0]; # 1