my (%symbol,@old,@new);

while (<DATA>) {
   chomp($_ = substr lc($_),0,2);
   push @old, $_;
   $symbol{$_}++;
   push @new, $_ . $symbol{$_};
}

print "$_\n" for @new;

__DATA__
Line one
Line two
xxxxxxxxxxx
Another line
xxxxxxxxxxx
Lines end here not
xxxxxxxxxxx
Fourth line (used to be)
Line five
Lines end here
Lines end here too