Hash is an associative array where data is stored in 
"key"->"value" pairs.

Eg : fruits is a hash having their names and price
%fruits = ("Apple", "60", "Banana", "20", "Peers", "40");

Hash/Associative array is the one in which all
the values are stored in terms of keys and values.

i.e., the values are indexed with the help of keys.
You cannot get back the values in a correct order as it 
happens in normal array. because in this values are stored 
in random positions. The only way to get in a correct order 
is to sort that array.

ex: %hash = {"Name","Raghav","Age","26"};
$hash{'Name'} will give "Raghav"
$hash{'Age'} will give "26"

Hash is a specific datatype in PERL and it contains KEYS 
and VALUES. Values are referred with the help of keys.

Hash or "Associative Array" is an unordered list of scalar 
data. The data of the hash is depicted as key and a value 
pair.

Eg: %h=("kar","blr","ap","hyd","tn","che");

To print the entire hash : print %h; --returns entire hash 
(the order may be different)
To print only one value : print $h{"kar"}; -->returns blr

As friends said above, its otherwise called as associative
array. It has lot of advantages over perl.

Here we can keep the values in a structured way, that
structured way comes from key-value pairs.

Eg: (The syntax given above are also correct, but the below
representation is much better in look and feel)

my %hash_example = ( a => 10,
                     b => 20,
                     c => 30 );  
keys   : a,b,c
values : 10,20,30

Features of Hash :
##################
1) Keys should always be unique where as values may not be
unique

(Right)
my %hash_example = ( a => 10,
                     b => 10,
                     c => 10 );

(wrong)
my %hash_example = ( a => 10,
                     a => 20,
                     a => 30 );

2) keys (%hash_example) will return an array containing only
keys

my @keys = keys(%hash);

3) similary for values
my @values = keys(%values);