Answer.

History of the question:

Perl has long been a choice for programmers for text and data processing due to its expressiveness and "magical" memory management capabilities. With the emergence of complex data structures and the active use of references, the need to understand how Perl manages memory became critical for supporting stable and high-performance scripts.

Problem:

In the standard memory management model, Perl uses reference counting: each object or variable in memory tracks how many references exist to it. When the last reference to an object disappears, the memory for it is automatically freed. However, the introduction of structures where elements reference each other (e.g., mutual or circular references) can lead to situations where memory is not freed at all. This causes memory leaks, which is particularly problematic in long-lived processes and when working with large arrays or hashes of complex nesting.

Solution:

Perl solves most memory management issues through a reference counting system, and to combat cycles, it is recommended to use weak references via the Scalar::Util module. It is also important to manually break cycles where automatic means do not suffice.

Example:

use Scalar::Util qw(weaken);

my $parent = {};
my $child = { parent => $parent };
$parent->{child} = $child;
weaken($child->{parent}); # break the cycle

Key features:

Perl removes objects immediately when the reference count drops to zero.
Cyclic references are not automatically removed without weakening.
Utility modules (e.g., Scalar::Util) help break cycles for proper memory release.

Tricky questions.

What happens if a circular reference is not broken and the script ends?

Answer: Upon the script's termination, the operating system will free all occupied resources, but in long-lived processes (daemons, servers), this will lead to accumulating unreleased memory within the process.

If a variable is assigned undef, will all memory be freed?

Answer: Only if there are no other active references to the object.

Example:

my $ref = [];
my $alias = $ref;
undef $ref; # alias still holds a reference – memory is not freed

Can memory leak even without circular references?

Answer: Yes, if references continue to exist due to, for example, global variables, closures, or non-specifically cleared arrays/hashes.

my $glob = [];
sub hold { $glob }
# $glob is not cleared — always holds data

Common mistakes and anti-patterns

Unintentionally creating circular references (parent-child objects).
Using global variables to store large data.
Careless use of closures that hold references.

Real-life example

Negative case

A web script stores user sessions in large structures that contain circular references between objects but does not use weaken. Sessions are not cleared, and memory keeps growing.

Pros:

Convenient to implement logic through parent-child relationships.

Cons:

Memory not being freed leads to server crashes/slowdowns.

Positive case

A script uses Scalar::Util::weaken for parent-references or manually breaks cycles at the end of session work.