davidos code is way more elegant, if you employ matijas fix, but since I wrote it, here is a rather ugly solution: 

my %hash2 = ();
my %unique = ();
foreach my $test (grep(!$unique{$_}++, values %hash)) {
    $hash2{$test} .= join(",", grep({$hash{$_} eq $test ? hash{$_} : u
+ndef} keys %hash));
}

[download]

And since I was wondering, it isn't slower than davidos solution, the solutions seem to be equally efficient: 

use strict;
use warnings;
use Data::XDumper qw(Dump);
use Benchmark qw(:all);
my %hash = (
    ball1 => 'red',
    ball2 => 'blue',
    ball3 => 'red',
);

print scalar Dump(\%hash);
                my %hash2 = ();
                my %unique = ();
                my %reverse_hash;

cmpthese(1000000, {
        'unique_grep' => sub {
                %unique = ();
                %hash2 = ();
                foreach my $test (grep(!$unique{$_}++, values %hash)) 
+{
                    $hash2{$test} .= join(",", grep({$hash{$_} eq $tes
+t ? $hash{$_} : undef} keys %hash));
                }
        },
        'reverse_hash' => sub {
                %reverse_hash = ();
                while ( my ( $key, $value ) = each %hash ) {
                    push @{$reverse_hash{$value}}, $key;
                }
        },
});
__END__
1st run:
                Rate  unique_grep reverse_hash
unique_grep  29940/s           --          -2%
reverse_hash 30694/s           3%           --

2nd run:
                Rate reverse_hash  unique_grep
reverse_hash 28466/s           --          -2%
unique_grep  29104/s           2%           --

[download]

regards,
tomte

Hlade's Law:

If you have a difficult task, give it to a lazy person --
they will find an easier way to do it.

In reply to Re: mildly tricky hash question by Tomte
in thread mildly tricky hash question by Anonymous Monk

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