my %trie;
%trie = (
  1 => { 1 => 'accept', 
         2 => {
                2 => 'accept',
                3 => \%trie{2}->{3}, # since we already saw a 23
                _default => \%trie,
         }
         _default => \%trie },
  2 => { 3 => { 
                3 => 'accept',
                _default => \%trie{3}, # we already saw a 3
              },
         _default => \%trie
       },
  3 => { 1 => 'accept', _default => \%trie },
  _default => \%trie;
);

# hash walker omitted