while ($line = <$inputFileHandle>)
{
  if ($sizeSoFar > 1e9 and $line =~ /^100/)
  {
    $outputFileName++;
    open $outputFileHandle, '>', $outputFileName or die "Cannot open $outputFilename for writing: $!\n";
    $sizeSoFar = 0;
  }
  print $outputFilehandle $line;
  $sizeSoFar += length($line);
}