OK, I ran the following code

# you can use print for debugging purposes, e.g.
# print "this is a debug message\n";
use Data::Dumper;

sub solution {
    my ($L)=@_;
    print Dumper $L;
    # write your code in Perl 5.14
}
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and got this output, which clarifies that the implementation is done with hashes of hashes

(sorry indentation lost while copying)

Running solution...

Compilation successful.

Example test:   [1, 1, 1, 1]
Output (stderr):
WARNING: producing output may seriously slow down your code!
Output:
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$VAR1 = {
          next => {
                      next => {
                                  next => {
                                              next => undef,
                                              value => 1
                                            },
                                  value => 1
                                },
                      value => 1
                    },
          value => 1
        };

Reading the confusing text reveals that those lazy people didn't even try to use proper Perl terminology.

Pointer? Dictionary? Empty? WTF?