#!/usr/bin/perl --
# filter.pl

use strict;
use warnings;

die "Usage: perl filter.pl pe_real_sample.txt > last.output\n"
  unless @ARGV;

my %seen;
my @ary;

# push into @ary every line which have not be seen yet

while (my $line = <>) {
    $line =~ /[fr] / or die "Can't parse line: $line";
    my $key = $` . $';
    push(@ary, [$line, $key]) unless $seen{$key}++;
}

# print all lines from @ary having $seen only once

print $_->[0] foreach grep {$seen{$_->[1]} < 2} @ary;
[download]

This script can be written more shortly:

#!/usr/bin/perl -n
/[fr] /, $s{$`.$'}++ || push @a, [$_,$`.$'] }{ $s{$_->[1]}<2 && print 
+$_->[0] for @a
[download]

or even:

#!/usr/bin/perl -n
/f|r/,$s{$`.$'}++,push@a,[$_,$`.$']}{$s{$$_[1]}<2&&print$$_[0]for@a
[download]