open (FILE,"<bigfile.log");
while ($line = <FILE>) {
  $count++;
}
$numlines = $count;
$count = 0;
close FILE;

open (FILE,"<bigfile.log");
open (SMALLERFILE,">smallerfile.log");
while ($line = <FILE>) {
  $count++;
  if ($count > $numlines - 1000) {
    print SMALLERFILE $line;
  }
}
close FILE;
close SMALLERFILE;