How do I look up a hash element by value
Get the Key corresponding to any given value

I assume you meant "I know that I can get the value for a certain key if it is the only value for that key".

There are two methods to store more than one value per key: The simple one using string concatenation of the different values and a more sophisticated one where you store a link to an array as hash value. Here is how to do it with string concatenation:

$hash{$key}= 'ape:b:cat' #stores the values 'ape','b' and 'cat' into t
+he hash using ':' as delimiter
#simple but the delimiter better not be in any values (you would have 
+to escape the delimiter which is messy)

@values= split /:/,$hash{$key}; #get the values out of the string
# checking for a specific value 
if ($hash{$key}=~/(^|:)$whatimlookingfor(:|$)/) {
  print "found it"; 
}

$hash{$key}.= ':' . $anothervalue; #add another value (but if the hash
+ value was empty you have just added two values, an empty string and 
+$anothervalue. This is another disadvantage)
[download]

The other method uses arrays to store the values. The structure is commonly called HashofArrays (HoA). The syntax is a bit more complicated, but it avoids some disadvantages of the first method

$hash{$key}= ['ape','b','cat']; #stores the values 'ape','b' and 'cat'
+ into the hash

@values= @{$hash{$key}}; #get all the values
$value= $hash{$key}->[2]; #get the third value
$value= $hash{$key}[2]; #get the third value, short form

push @{$hash{$key}}, $anothervalue; #add a value
[download]

As you can see the second method should be prefered, but if the syntax of a HashofArrays looks daunting to you or you have other compelling reasons, the first method can be used as well

Values should be unique for each and every key while getting the hash key using value(For this cenario). Otherwise you may not get the key which you want.

For example if you are having "apple" as a value in the h1 and h2 key. while getting the key using value(apple) you can't get your actual key which you want.

use strict;
use warnings;
use Data::Dumper;

# %data is an example Hash-of-Arrays structure
# where the hash has 3 keys (a,b,c) and each
# key has 3 values:
my %data = (
    a => [1..3],
    b => [4..6],
    c => [7..9],
);
#print Dumper(\%data);

print get_key(3), "\n";
print get_key(8), "\n";
print get_key(0), "\n";

sub get_key {
    my $value = shift;
    for my $key (keys %data) {
        for my $val (@{ $data{$key} }) {
            if ($val eq $value) {
                return $key;
            }
        }
    }
    return 'key not found';
}

__END__

a
c
key not found
[download]

Another way to think about it is: if you are having to look up values to find keys you have built your hash the wrong way aroung. Hashes are good looking up keys to find values but not so good for looking up values to find keys. So, when you create your hash, swap the keys and values. Then your lookup will be easy.

You can try something like the following: