Actually it's just:

$data[$i] - $data[$i - 1]
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What though have you actually tried? What error message or behaviour is confusing you? Can you put together a sample something like the following code that demonstrates the problem you have?

use strict;
use warnings;

my @table = (1, 2, 4, 6, 10);
my @diffs;

for my $index (0 .. $#table - 1) {
    $diffs[$index] = $table[$index + 1] - $table[$index];
}

print "@diffs\n";
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