Your biggest (bug causing) problem is the use of:

 length[@data_array]
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instead of

 length (@data_array)
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There are other style-related issues, primarily dealing with localizing the scope of declared variables, and using a "c-style" "for" loop where a "perl style" would be more appropriate. Let me know if you need elaboration on either point.

UPDATE: length (@data_array) will not give you expected results either . (Thanks choroba for pointing that out) ; use

scalar(@data_array)
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instead.

This is because length() takes a scalar. These statements are equivalent (return the same value):

      length (@data_array) ==  length (scalar @data_array)
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So the value returned by length is the length in bytes of the size of the array.
I.e. if there are 900 elements in the array, length() would return "3" (Whereas scalar() would return 900).