Wow, you don't learn do you.
Do you mean that you need to merge two files by printing alternate lines from each?

To do that you could

• set a $data_left flag to 3
• open both files, checking your opens succeeded
• while the $data_left flag is true
  • if you can read from the first file and $data_left%2 = 1
    • print out the line you read
  • if however you couldn't read from the first file and $data_left%2 = 1
  • set the $data_left flag to $data_left - 1
  • if you can read from the second file and $data_left >= 2.
    • print out the line you read
  • if however you couldn't read from the second file and $data_left >= 2
  • set the $data_left flag to $data_left - 2
• close the files

This assumes you want to continue adding data after one of the files is empty and that you don't want to insert blank lines as placeholders for the empty file