use Data::Alias qw( alias );

sub find_undef_element {
   my ($hash) = @_;
   while (my ($key, $val) = each(%$hash)) {
      if (!defined($val)) {
         keys(%$hash);  # Reset iterator.
         return $key;
      }
   }

   return undef;
}

my $addr = 0x1234;
my $ident = "abc";

my %by_addr;
my %by_ident;

$by_addr{$addr} = undef;
$by_ident{$ident} = undef;
alias my $shared_addr = find_undef_element(\%by_addr);
alias my $shared_ident = find_undef_element(\%by_ident);

alias $by_addr{$shared_addr} = $shared_ident;
alias $by_ident{$shared_ident} = $shared_addr;