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.

In reply to Re^3: print a determinate line number by arun_kom
in thread print a determinate line number by Paulux

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