# define these outside the loop, please.
  my @chars = ( "A" .. "Z", "a" .. "z", 0 .. 9, qw(! @ $ % ^ & *) );

  # load the previously generated passwords from the file:
  my $usedpw = "logfile.txt" ;
  open USEDPW, "< $usedpw" or die "Error opening $usedpw for reading: $!\n";
  flock USEDPW, 1; # shared lock
  my @used_pw = <USEDPW>;
  close USEDPW;
  chomp @used_pw;
  # and put them in a set (implemented as a hash):
  my %used_pw;
  for ( @used_pw ) { $used_pw{$_}++ }

  my $pw;
  do
  { # create a password:
    $pw = join '', @chars[map{rand @chars} (1..17)];
  }
  while exists $used_pw{$pw};

  # we now have a password which has not been used before, so report it
  print "Your unique password is: $pw\n";
  # and store it in the file.
  open USEDPW, "> $usedpw" or die "Error opening $usedpw for writing: $!\n";
  flock USEDPW, 2; # exclusive lock 
  print USEDPW "$pw\n";
  close USEDPW;