Here are some notes to help you understand get_line:

• !defined($buf) is only true the first time get_line is called. It proceeds to trash all the lines before the first timestamped line, if any.

• The file is processed as follows:

  File                Read by      Returned by
  -----------------   -----------  -----------
  line                1st call(*)  scrapped
  line                1st call(*)  scrapped
  timestamped line    1st call(*)  1st call
  line                1st call     1st call
  line                1st call     1st call
  timestamped line    1st call     2nd call
  line                2nd call     2nd call
  line                2nd call     2nd call
  timestamped line    2nd call     3rd call
  line                3rd call     3rd call
  line                3rd call     3rd call
  timestamped line    3rd call     4th call
  EOF                 4th call     5th call
  [download]

  * — By the body of if (!defined($buf)).

• Between calls, $buf contains the line that has been read, but not returned.

• our $var; local *var = \($self->[$idx]);
  creates an alias so that any change to $var is reflected in $self->[$idx].

• for (;;) can be read as "for ever". The loop will loop until last, return, die, exit or other exceptional means are used to exit it.

• return ((undef, $buf) = ($buf, $line))[0];
  is short for
  my $temp = $buf;
  $buf = $line;
  return $temp;