$line_0 = "";
$line_1 = "";
$line_2 = "";
$line_3 = "";

while ($line = <FILE>)
{
  chomp $line;
  $line_3 = $line_2;
  $line_2 = $line_1;
  $line_1 = $line_0;
  $line_0 = $line;
}