my @a;

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

##</code><code>##

use v5.12;
use warnings;

my (@x,%x);

my $i=1;
for my $v (undef,0,7) {
  $x[$i]=$x{$i}=$v;
  $i++;
}

##</code><code>##

--- Index exists?
$i 	@x 	%x
0 	 	
1 	1 	1
2 	1 	1
3 	1 	1
4