Programming Perl, 4th Edition

Garbage Collection with destroy Methods

As described in the section Garbage Collection, Circular References, and Weak References in Chapter 8, a variable that refers to itself (or multiple variables that refer to one another indirectly) will not be freed until the program (or embedded interpreter) is about to exit. If you want to reclaim the memory any earlier, you usually have to explicitly break the reference or weaken it using the Scalar::Util module.

With objects, an alternative solution is to create a container class that holds a pointer to the self-referential data structure. Define a DESTROY method for the containing object’s class that manually breaks the circularities in the self-referential structure. You can find an example of this in Recipe 13.13, “Coping with Circular Data Structures Using Weak References” of Perl Cookbook.

When an interpreter shuts down, all its objects are destroyed, which is important for multithreaded or embedded Perl applications. Objects are always destroyed in a separate pass before ordinary references. This is to prevent DESTROY methods from using references that have themselves been destroyed. (And also because plain references are only garbage collected in embedded interpreters, since exiting a process is a very fast way of reclaiming references. But exiting won’t run the object destructors, so Perl does that first.)