in reply to Merge the difference between two files and save it on the first file. Also update the first file with info in second file.
read the first file line by line. Split the line at the '='. Take the first part as key of a hash and the second as value of that key. Then do the same for the second file. When you hit in the second file a key which was already read than the value gets substituted. When you have read both files print every tuple (key, value) of your resulting hash to the first file.
When you have problems with some of that subtasks show us your code and we'll help you.