open BIGFILE, $filename or die;
while (<BIGFILE>)
{
  if (/(.+)$/ ... /^\$$/ && next)
  {
    if (defined $1) { open SMALLFILE, ">$1" or die; next }
    print SMALLFILE;
    next;
  }
  warn "Untested code is bad!";
}
close BIGFILE;