Intermediate Perl, 2nd Edition

Chapter 5. References and Scoping

We can copy and pass around references like any other scalar. At any given time, Perl knows the number of references to a particular data item. Perl can also create references to anonymous data structures that do not have explicit names and create references automatically as needed to fulfill certain kinds of operations. We’ll show you how to copy references and how it affects scoping and memory usage.

More than One Reference to Data

Chapter 4 explored how to take a reference to an array @skipper and place it into a new scalar variable:

my @skipper = qw(blue_shirt hat jacket preserver sunscreen);
my $ref_to_skipper = \@skipper;

We can then copy the reference or take additional references, and they’ll all refer to the same thing and are interchangeable:

my $second_ref_to_skipper = $reference_to_skipper;
my $third_ref_to_skipper  = \@skipper;

At this point, we have four different ways to access the data contained in @skipper:

@skipper
@$ref_to_skipper
@$second_ref_to_skipper
@$third_ref_to_skipper

Perl tracks how many ways it can access the data through a mechanism called reference counting. The original name counts as one, and each additional reference that we create (including copies of references) also counts as one. The total number of references to the array of provisions is now four.

We can add and remove references as we wish, and as long as the reference count doesn’t hit zero, Perl maintains the array in memory, and it is still accessible via any of ...