Shell Programming and Scripting

Below is one article I was reading and I don't understand the

Code:

$seen{$_} = 1

Can someone explain this in easier terms plesae?

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We can also take advantage of the uniqueness property of hash keys to filter out duplicates from a list. For example, suppose we need to examine lines of a file, generating an output that consists of only one line even if the line is duplicated. Easy enough:

Code:

  %seen = ();
  while (<>) {
    $seen{$_} = 1;
  }
  foreach (sort keys %seen) {
    print;
  }

Code:

$seen{$_} = 1

Each an every line from the while loop(i.e., $_) is considered as values of a hash and are assigned to 1.

thank u. .i understand now

Actually its the key - value representation that is made to the hash !
key = $_
value = 1