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my @a;

$a[1] = undef;
$a[2] = 0;
$a[3] = 7;

use v5.12;
use warnings;

...
  $x[$i]=$x{$i}=$v;
  $i++;
}

--- Index exists?
$i     @x     %x
...
2     1     1
3     1     1
4