You are trying to access an element which is beyond the last one in your array: 
$file_array[$i+3]

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You have 20 elements in the array (20 lines in your input), and the last time through the loop, $i=18, but $i+3=21.

Tip #2 from the Basic debugging checklist : add print "$i\n" near the top of your loop.

In reply to Re: Use of uninitialized value in concatenation (.) or string by toolic
in thread problem in for loop by MVRS

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