Perl - References

A Perl reference is a scalar data type that holds the location of another value which could be scalar, arrays, or hashes. Because of its scalar nature, a reference can be used anywhere, a scalar can be used.

You can construct psts containing references to other psts, which can contain references to hashes, and so on. This is how the nested data structures are built in Perl.

Create References

It is easy to create a reference for any variable, subroutine or value by prefixing it with a backslash as follows −

$scalarref = $foo;
$arrayref  = @ARGV;
$hashref   = \%ENV;
$coderef   = &handler;
$globref   = *foo;

You cannot create a reference on an I/O handle (filehandle or dirhandle) using the backslash operator but a reference to an anonymous array can be created using the square brackets as follows −

 $arrayref = [1, 2, [ a ,  b ,  c ]];

Similar way you can create a reference to an anonymous hash using the curly brackets as follows −

$hashref = {
    Adam   =>  Eve ,
    Clyde  =>  Bonnie ,
};

A reference to an anonymous subroutine can be created by using sub without a subname as follows −

$coderef = sub { print "Boink!
" };

Dereferencing

Dereferencing returns the value from a reference point to the location. To dereference a reference simply use $, @ or % as prefix of the reference variable depending on whether the reference is pointing to a scalar, array, or hash. Following is the example to explain the concept −

#!/usr/bin/perl

$var = 10;

# Now $r has reference to $var scalar.
$r = $var;

# Print value available at the location stored in $r.
print "Value of $var is : ", $$r, "
";

@var = (1, 2, 3);
# Now $r has reference to @var array.
$r = @var;
# Print values available at the location stored in $r.
print "Value of @var is : ",  @$r, "
";

%var = ( key1  => 10,  key2  => 20);
# Now $r has reference to %var hash.
$r = \%var;
# Print values available at the location stored in $r.
print "Value of %var is : ", %$r, "
";

Value of 10 is : 10
Value of 1 2 3 is : 123
Value of %var is : key220key110

SCALAR
ARRAY
HASH
CODE
GLOB
REF

#!/usr/bin/perl

$var = 10;
$r = $var;
print "Reference type in r : ", ref($r), "
";

@var = (1, 2, 3);
$r = @var;
print "Reference type in r : ", ref($r), "
";

%var = ( key1  => 10,  key2  => 20);
$r = \%var;
print "Reference type in r : ", ref($r), "
";

Reference type in r : SCALAR
Reference type in r : ARRAY
Reference type in r : HASH

#!/usr/bin/perl

 my $foo = 100;
 $foo = $foo;
 
 print "Value of foo is : ", $$foo, "
";

Value of foo is : REF(0x9aae38)

#!/usr/bin/perl

# Function definition
sub PrintHash {
   my (%hash) = @_;
   
   foreach $item (%hash) {
      print "Item : $item
";
   }
}
%hash = ( name  =>  Tom ,  age  => 19);

# Create a reference to above function.
$cref = &PrintHash;

# Function call using reference.
&$cref(%hash);

Item : name
Item : Tom
Item : age
Item : 19