use Data::Dump "pp";
my %hash = ('one' => '1', 'two' => '2','1'=>'one');
my %seen;
for my $key ( keys %hash )
{
    delete $hash{$key} if $seen{$key} || $seen{$hash{$key}};
    $seen{$key}++;
    $seen{$hash{$key}}++;
}
pp \%hash;
--
{ 1 => "one", two => 2 }