# you'll need to provide $filename
open(my $fh, '+<', $filename)
    or die "Error opening $filename: $!";
# create a scope to localize the input record separator
{
  # use formfeed as input record separator
  local $/ = "\x{0c}";
  # track position of start/end of current record
  my($start_pos, $end_pos) = (undef, 0);
  while (defined(my $record = <$fh>)) {
    ($start_pos, $end_pos) = ($end_pos, tell($fh));

    # placeholder: decide if this record needs to be changed
    next unless needs_change($record);

    # placeholder: work out what change should be made
    my($offset_to_write_at, $text_to_write) = required_change($record);

    seek($fh, $start_pos + $offset_to_write_at, 0)
        or die "Error seeking to $start_pos + $offset_to_write_at";
    print $fh, $text_to_write or die "Error writing update";
    # ready to read next record 
    seek($fh, $end_pos, 0) or die "Error seeking to $end_pos";
  }
}
close $fh or die "Error closing filehandle, writes may not have completed";