Update My apologies -- I completely missed the line about "no arrays or hashes" -- sorry for the noise

This way has always worked for me:

use strict;
use warnings;
use Data::Dumper;

my %hash;

open(IFH, "<", "data.txt");
while(<IFH>) {
  chomp;    

  # keep a running count of occurrences for each line string
  $hash{$_}++;
}  
close IFH;

my @uniq = sort grep { $hash{$_} == 1} keys %hash;
print Dumper(\%hash);
print Dumper(\@uniq);


__OUTPUT__

$VAR1 = {
          'j1j' => 1,
          'i1i' => 1,
          'b1b' => 1,
          'a1a' => 2,
          'f1f' => 1,
          'e1e' => 1,
          'h1h' => 2,
          'c1c' => 2,
          'g1g' => 2,
          'd1d' => 2
        };
$VAR1 = [
          'b1b',
          'e1e',
          'f1f',
          'i1i',
          'j1j'
        ];
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