Programming Perl in C

Objects start their lives with a refcount of 1. When a reference to the object is 

created, its refcount is incremented. When an object goes out of scope, its refcount 

is decremented. Finally, when a reference is removed, the refcount of the object is 

decremented. The object is freed when its refcount reaches 0.

Most variables don't get referenced and simply go from having a refcount of 

1 to having a refcount 0 when they go out of scope:

{

   my $a = "foo";     # $a has a refcount of 1

}                    

# $a goes out of scope and has its refcount decremented.  Since its refcount is

# now 0, the memory used by $a is freed.

Even when references are created they are normally confined to a single scope:

{

   my $a = "foo";    # $a has a refcount of 1

   my $b = \$a;      # $b has a refcount of 1, $a has a refcount of 2

}

# $a and $b go out of scope and have their refcounts decremented.  Since $b

# referenced $a, $a has its refcount decremented again.  Now both $a and $b

# have refcounts of 0 and their memory is freed.

Things start getting complicated when an object is referenced by a variable 

from another scope. Here's a simple example:

my $a;               # $a has a refcount of 1

{

   my $b = "foo";    # $b has a refcount of 1

   $a = \$b;         # $a now references $b.  $b has a refcount of 2

}             

# $b goes out of scope. $b has its refcount decremented and now has a

# refcount of 1.  $b is still live since $a has a reference to it.

$a = 10;  # $a no longer references $b.  $b now has a refcount of 0 and is

          # freed by the garbage collector.

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