This is only a trace you can follow:

• read the second file, parsing each line (split) and populating a hash with var=>value
• open a temp file in write mode (File::Temp)
• read the first file and, line by line:
• extract the left term with a regex
• use it as a key to the hash
• retrieve the value
• substitute it in the line with another regex
• write the new line to a temp file
• on end of file, close the files and copy the temp file back in place of the first file (File::Copy)