my %config_settings = (
  CONFIG_NET => 'y',
  CONFIG_HOTPLUG => 'y',
  # whatever else you want to set
)

open(KERNEL_CONFIG, '<', '/usr/src/linux2.4/.config');
open(NEW_CONFIG, '>', '/usr/src/linux2.4/.config.new');
while my $config_line (<KERNEL_CONFIG>) {
  $config_line =~ /([A-Z0-9_])/;
  $config_line = $1 . '=' . $config_settings{$1} . "\n" if exists $config_settings{$1};
  print NEW_CONFIG $config_line;
}
close KERNEL_CONFIG;