# NOTE: this code is untested

chopf('/path/to/log.dat', 1000000);

sub chopf{
  my ($file, $size) = @_;
  local *HEAD;
  local *TAIL;
  return 1 unless -s $file > $size;
  return 0 unless open HEAD, "+<$file";
  unless (open TAIL, "+<$file"){
    close HEAD;
    return 0;
  }
  seek TAIL, -s($file) - $size, 0;
  <TAIL>;
  print HEAD $_ while (<TAIL>);
  truncate HEAD, tell(HEAD);
  close HEAD;
  close TAIL;
  return 1;
}