If you don't understand a data structure, you should use Data::Dumper.

If the dictionary contained

act
cat
dog
god
zebra
[download]

It builds the following hash

%dict = (
    act   => [ 'act', 'cat' ],
    dgo   => [ 'dog', 'god' ],
    aberz => [ 'zebra'      ],
);
[download]

If the user enters dog, it searches for dgo (hash("dog")) and lists what it finds (dog and god).