my $filename = "anything.ldif";

unless (open (LDIF, $filename)) {
  die "Error: couldn't read from '$filename': $!\n";
}
else {
  my %seenObjects = ();

  local $/ = "\n\n"; # read objects, not lines

  while (<LDIF>) { # $_ has one object now
    chomp($_);
    s/\n //g; # kill continuation lines

    if ($seenObjects{$_}++) {
      # object already seen... write to another filehandle
      print OUTFILE "$_\n\n";

    } # if

  } # while
  close (LDIF);
} # else