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'
                      ]
        };
...