ah i see it now ;) ... something like this then.

use strict;
use warnings;

my $previous_line;

while(<DATA>){
  chomp;
  if($.==1){
    $previous_line = $_;
    next;
  }
  if($_ eq $previous_line){ print "Line $. : $_\n"; }
  $previous_line = $_;
}

__DATA__
a
b
b
c
c
c
d
[download]

UPDATE:

Sorry the code above prints the duplicated lines, which is the opposite of what you wanted. i see that almut already has an elegant solution for you.