Shall we dance, sweet reference?

You now command two vital pieces of knowledge for working with Perl references: how to construct a reference, and how to dereference. Like learning how to juggle a hackeysack with your ankle, the question becomes clear: why do I want to do this anyway? What are these references good for?

The trick is, you've already used them, if you followed along last month's installment working with complex lists and hashes. In that article, we relied on anonymous references to construct nested lists and hashes. An anonymous reference is simply a reference that points to explicit data rather than a containing variable; for instance:

$days=["monday","tuesday","wednesday"];

Here, the reference $days points directly to the values listed, without being bound to an intermediary variable. We used this type of anonymous reference, for example, when we nested a list into a list:

push(@carmodels,["sentra,stanza,altima"]);

Similarly, you can create a reference that points to an anonymous hash:

$grades={"Tom Jones"=>92,"Briget 
Gidget"=>85};

It's very important to distinguish the above, where $grades is a reference pointing to a hash, from a "real" hash, which would not use the curly braces in its definition but rather parentheses. Thus, to access the value for the key "Tom Jones" in the anonymous hash, you'd use dereferencing syntax:

print $grades->{"Tom Jones"};

Now, let's twist. Because a reference can be treated as a scalar variable, you can use a reference in many places where you would use a scalar; for instance, a reference can be the value of a list item, or the value of a hash key. Keep the $grades reference in mind as we two-step through the following examples:

@class=("Physics 101","Exam 
1",$grades);

The third item in the list @class is now the reference $grades, which points to the anonymous hash seen previously. To access "Tom Jones" from this list:

print @class[2]->{"Tom Jones"};

Similarly, you can use a reference as the value of a hash key, but you cannot use a reference as a key of a hash.

%roster=("class"=>"Physics 
101","exam"=>1,"grades"=>$grades);

print $roster{"grades"}->{"Tom Jones"};

Dereference, my dear
The Perl You Need to Know
Take my Parameters, Please