my %dmap = (
  1 => undef,
  2 => 4,
  3 => 5,
  4 => 3,
  5 => 1,
);

my $key = 0;
defined $dmap{$_} or $dmap{$_} = $key for keys %dmap;

my %pamd = reverse %dmap;
print $key = $pamd{$key} while $pamd{$key};