The problem with filesystems is that files are treated as streams of data, instead of linked lists of lines. If files were linked lists, then insertion and deletion would be a snap, as well as traversing the list to a certain line.

As it is, the fastest way is to read X bytes from the end of the file, and check for the right-most \n character (that is NOT at the end of the string).

sub last_line {
  my ($fref, $r_len) = @_;
  my ($last, $chomped) = ("", 0);
  $r_len = -$r_len if $r_len > 0;
  $r_len ||= -80;

  seek $fref, $r_len, 2;
  {
    my ($buffer,$pos);
    read $fref, $buffer, $r_len;
    $last = $buffer . $last;
    if (($pos = rindex($last,"\n",length($last)-2)) != -1) {
      return substr($last,$pos);
    }
    seek $fref, $r_len * 2, 1;
    redo;
  }
}
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$_="goto+F.print+chop;\n=yhpaj";F1:eval