I think you want to replace

  @datapoint = splice(@values, 0, $x);
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with

  push @datapoint, splice(@values, 0, $x);
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That is, you want to add a new entry to the datapoint array.

While you're at it, you might want to

• use strict; use warnings;
• my your variables
• test the error returns from your unlink and second open
• use $i++; rather than $i += 1;