Hi,
I have a text file with a very long string of characters, like "ACGCTTGCAA...", and I want to make another file with 36 characters from this string file every 4 lines like:

character of position 1 to position 36 of the string file, position 2-37, position 3-38 ... position length((string)-35)-length(string).
And in the other lines of the file write always the same strings like below:

@read
AACCCAGTAAGTCACAGTGCANCAAACTTGTTTATT
+read
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@read
ACCCAGTAAGTCACAGTGCANCAAACTTGTTTATTC
+read
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@read
CCCAGTAAGTCACAGTGCANCAAACTTGTTTATTCA
+read
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

So my question is, how can I do it?

The short answer is:

perldoc -f substr

The long answer is: What have you tried so far?