while (<>) {
  print;
  if  (m/TheLineBefore/){
    # read the line to be replaced
    my $line = <>;
    # do something with it
    $line =~ s/1/0/;
    # print the new version
    print $line;
 }
}
__DATA__
test data
more
TheLineBefore
LocalCompilerPresent=1
more stuff