Do you mean "Given a value, how can I find the key?". If so the short answer is: "Probably you can't".

The longer answer is that keys are unique for any given hash but values are not. For example a hash may contain the following key => value pairs:

apple  => red
ball   => red
lemon  => yellow
lime   => green
grass  => green
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The keys are unique, but the values (colours) are not. You can not ask for the single green object because there is more than one.

If there were a one to one mapping (keys and values unique) then you could generate a second hash using the first hash's values as keys and the first hash's keys as values - you would then have a reverse lookup.

If however you are asking "How do I get a list of the keys", the answer is trivial: keys.

You could try something like this:

#!/usr/bin/perl -l
# vi:ts=4 sw=4 et:
use strict;
use warnings;

my %hash = (
    apple   => 'red',
    ball    => 'red',
    lemon   => 'yellow',
    lime    => 'green',
    grass   => 'green',
);

my $search = 'green';

# get all keys, which have $search as value
my @keys = grep { $hash{$_} eq $search } keys %hash;

print "@keys";
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Note that keys are unique but values can be not unique. So if different keys have the same value how do you want to reverse them?

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



my %hash = (
    apple   => 'red',
    ball    => 'red',
    lemon   => 'yellow',
    lime    => 'green',
    grass   => 'green',
);
my %rev;



#= version 1
%rev=();

for my $value (values %hash) {
    $rev{$value}=[ grep { $hash{$_} eq $value } keys %hash ];
}

print Dumper \%rev;


#= version 2

%rev=();

while (my ($k,$v) = each %hash) {
    push @{ $rev{$v} }, $k ;
}

print Dumper \%rev;



#= version 2b
# not really shorter under strict ...

%rev=();

my ($k,$v);
push @{ $rev{$v} } , $k
    while ( ($k,$v) = each %hash);


print Dumper \%rev;


#= version 2c
# hmm maybe finally a good application of builtin pkgvars $a and $b  ;
+)
# VIVA LA AUTOVIVICACION! 8 )

%rev=();


push @{ $rev{$b} } , $a
    while ( ($a,$b) = each %hash);


print Dumper \%rev;

__END__ #OUTPUT
$VAR1 = {
          'green' => [
                       'grass',
                       'lime'
                     ],
          'red' => [
                     'apple',
                     'ball'
                   ],
          'yellow' => [
                        'lemon'
                      ]
        };
...
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