my %config = (
   CONFIG_PCI => 'y',
   CONFIG_BSD_PROCESS_ACCT => 'y',
);
while (<>) {
   if ((my ($n,v) = split/=/) == 2) {
      if (my $new_value = $config{$n}) {
         print "$n=$new_value\n";
         next;
      }
   }
   print;
}