if the following line is different from the previous i have to print it

For that, you'd have to keep the previous line:

my $prev;
while (<>) {
    print if defined($prev) and $_ ne $prev;
    $prev = $_;   # update $prev
}
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(The defined check would skip the first line, as there isn't really one previous to it...)

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
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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.

Thanx a lot, i have just added some control but it works.