Your code is highly confused. There only is one instance of the MYFILE filehandle and the flow is as follows:

1. while(<MYFILE>)... reads one line from the file into $_
2. print <MYFILE> reads all remaining lines from MYFILE and prints them
3. $data = join "", <MYFILE> assigns the empty string to $data because MYFILE is at the end of the file now.

Maybe you can take a step back and explain why you are sprinkling various attempts to read from MYFILE over your code and what your real goal is?