B.1 References

1. Scalar references:

   $ra  = \$a;              # reference to scalar
   $$ra = 2;                # dereference scalar-ref
   $ra  = \1.6;             # reference to constant scalar

2. Array references:

   $rl  = \@l;              # reference to existing
   $rl  = [1,2,3];          # reference to anon. scalar
   push (@$rl, "a");        # Dereference
   print $rl->[3]           # 4th element of array pointed to by $rl

3. Hash references:

   $rh = \%h;               # reference to hash
   $rh = {"laurel" => "hardy", "romeo" => "juliet"}; # ref to anon-hash
   print keys (%$rh);       # Dereference
   $x = $rh->{"laurel"};    # Arrow notation to extract single element
   @slice = @$rh{"laurel","romeo"}; # Hash slice

4. Code references:

   $rs = \&foo;             # reference to existing subroutine foo
   $rs = sub {print "foo"}; # reference to anonymous subroutine 
                            # (remember the semicolon at the end)
   &$rs();                  # dereference: call the subroutine

5. Generalized dereferences. Any code inside a block yielding a reference can be dereferenced:

   @a = @{foo()};           # dereference the array reference 
                            # returned by foo()

6. References gotchas. All the examples below are wrong. Always use -w in developing and testing.

   @foo = [1,3,4];          # Assigning an array-ref to an array
                            # Use parentheses instead.
   
   %foo = {"foo" => "bar"}; # Assigning a hash-ref to a hash.
                            # Use parentheses instead.
   
   $foo = \($a, @b);        # Identical to $foo = (\$a, \@b)
                            # Assiging an enumerated list to a 
                            # scalar yields the last element (so, 
                            # $foo gets \@b). Use [ ] if you need 
                            # an array reference