my %hash = ('apple2' => 'green',
            'apple1' => 'green',
            'banana' => 'yellow',
            'plum' => 'red');

my %distinct;
my %overflow;

while (my ($k, $v) = each %hash) {
# construct1
#   (defined $distinct{$v}) ? ($overflow{$k} = $v)
#                           : ($distinct{$v} = $k);
# construct2
   (exists $distinct{$v}) ? $overflow{$k} = $v
                          : $distinct{$v} = $k;
}
for (keys %overflow) {
   print "$_, $overflow{$_} \n";
}