Programming Perl, 4th Edition

Garbage Collection, Circular References, and Weak References

High-level languages typically allow programmers not to worry about deallocating memory when they’re done using it. This automatic reclamation process is known as garbage collection. For most purposes, Perl uses a fast and simple reference-based garbage collector.

When a block is exited, its locally scoped variables are normally freed up, but it is possible to hide your garbage so that Perl’s garbage collector can’t find it. One serious concern is that unreachable memory with a nonzero reference count will normally not get freed. Therefore, circular references are a bad idea:

{               # make $a and $b point to each other
    my ($a, $b);
    $a = \$b;
    $b = \$a;
}

or more simply:

{               # make $a point to itself
    my $a;
    $a = \$a;
}

Even though $a should be deallocated at the end of the block, it isn’t. When building recursive data structures, you’ll have to break (or weaken; see below) the self-reference yourself if you want to reclaim the memory before your program (or thread) exits. (Upon exit, the memory will be reclaimed for you automatically via a costly but complete mark-and-sweep garbage collection.) If the data structure is an object, you can use a DESTROY method to break the reference automatically; see Garbage Collection with destroy Methods in Chapter 12.

A similar situation can occur with caches—repositories of data designed for faster-than-normal retrieval. Outside the cache there are references to data inside the cache. The problem ...