my %reverse_hash;
foreach my $key ( keys %hash ) {
    push @{$reverse_hash{$hash{$key}}, $key;
}
[download]

Or maybe a little clearer:

my %reverse_hash;
while ( my ( $key, $value ) = each %hash ) {
    push @{$reverse_hash{$value}}, $key;
}
[download]

That would give him a hash of arrays - not what he's looking for. You could join all the arrays right after your loop, or you could change it to

my %reverse_hash;
foreach my $key ( keys %hash ) {
    $reverse_hash{$key}=$reverse_hash{$key}
                        ? "$reverse_hash{$key},$hash{$key}"
                        : $hash{$key};
}
[download]

I'm getting an "undefined" error at the print statement when I use that bit of code. Warnings says that line 16 is using an uninitilized value..... what have I done wrong?

#!/usr/bin/perl -w
use strict;

my $key;
my %hash = (
             ball1 => 'red',
             ball2 => 'blue',
             ball3 => 'red',
           );
# thanks to http://www.perlmonks.org/index.pl?node_id=340896 for this 
+block of code

my %reverse_hash;
foreach $key ( keys %hash ) {
    $reverse_hash{$key} = $reverse_hash{$key}
                        ? "$reverse_hash{$key},$hash{$key}"
                        : $hash{$key};
}
print "$reverse_hash{red}\n";
[download]

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: