in reply to
reading the 5th element from file doesn't work.
... what could be the issue here.
Allow Perl to suggest what the issue might be by using the warnings pragma, which would have warned you that $string was undefined (and if you're a beginner, also diagnostics for more verbose suggestions). In addition, I would strongly suggest using the strict pragma to disallow certain practices that experience has shown to be problematic, e.g., use of variables that have not been pre-declared. So:
use diagnostics; # when you are desperate