while (<INPUT>) {
  # look for start of record
  next unless /START_RECORD_MARKER/;

  # output the next 17 lines into a file
  my @seventeen;

  push @seventeen, scalar <INPUT> for 1 .. 17;

  # reformat the contents of @seventeen in some way

  open OUTPUT, '>', 'first_part_of_record.txt'
    or die $!;

  print OUTPUT @seventeen;

  close OUTPUT;

  # output the rest of the record into another file
  my @rest;
  while (<INPUT>) {
    last if /END_RECORD_MARKER/;

    push @rest, $_;
  }

  # reformat @rest in some way

  open OUTPUT, '>', 'next_part_of_record.txt'
    or die $!;

  print OUTPUT @rest;

  close OUTPUT;
}