let's say I opened a file, then parsed it into lines. Then I use a loop:

foreach line $lines {}

inside the loop, for some lines, I want to replace them inside the file with different lines. Is it possible? Or do I have to write to another temporary file, then replace the files when I'm done?

e.g., if the file contained

AA
BB

and then I replace capital letters with lower case letters, I want the original file to contain

aa
bb

Thanks!