use strict;
use warnings;

my @arr = ();

open(IFH, "<", "data.txt");

my $cur = scalar <IFH>;
push @arr, $cur;

# @arr contains, at most, N identical lines 
# .ie if "d1d" occurs five times in a row, then 
# @arr will contain the 5 occurrences of "d1d"

# @arr is reset to one element as new strings
# are encountered

while($cur = <IFH>) {
  if ($cur eq $arr[0]) {	
    push @arr, $cur;
  }  
  else {
    # if here, we have a new string, so check
    # the size of @arr to see if current string is unique
    print $arr[0] if scalar(@arr) == 1;
    @arr = ($cur);
  }	  
}  
print $arr[0] if scalar(@arr) == 1;

close IFH;

__OUTPUT__
b1b
e1e
f1f
i1i
j1j