Perl Module Basics

This example works similarly to the earlier example and uses an @ to deference 

the reference to @array. This works fine for access to the whole array, but more 

often you'll want to pick out a single value:

print $ref >[0] . "\n";  # prints "Hello,"

This syntax is known as arrow notation. You can use arrow notation with hashes 

as well. For example, here's another way to print  Hello, New World , this time 

using a reference to a hash:

%hash = ( message => "Hello, New World" );

$ref = \%hash;

print $ref >{message} . "\n";

Finally, Perl contains operators to create array and hash references without 

requiring an intermediate variable. These are known as anonymous arrays and 

anonymous hashes, respectively. For example, the preceding example could be 

rewritten to use an anonymous hash:

$ref = { message => "Hello, New World" };

print $ref >{message} . "\n";

The curly braces ({}) produce a reference to an anonymous hash. Similarly, 

square braces ([]) produce a reference to an anonymous array:

$ref = [ "Hello", "New", "World" ];

print join(" ", @$ref) . "\n";

References are often used to implement call by reference in subroutines. Call 

by reference means that the subroutine takes a reference as a parameter and acts 

on the data pointed to by the reference.  For example, here's a function that takes 

a reference to an array of words and uppercases them:

sub upper {

   my $words = shift;

   $words >[$_] = uc($words >[$_]) for (0 .. $#$words);

}

Notice that this subroutine doesn't return anything it works by modifying the 

array pointed to by the reference passed as the first parameter. Here's an example 

of how upper() would be used:

my @words = ("Hello,", "New", "World");

upper(\@words);

print join(" ", @words) . "\n";  # prints "HELLO, NEW WORLD"

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