It may be useful to you that an array element that has never been assigned to will return false when tested by exists (however, calling exists on array values is deprecated).

>perl -wMstrict -le
"my @ra;
 $#ra = 10;
 @ra[3, 5, 7] = (33, 55, 77);
 ;;
 use Data::Dumper;
 print Dumper fetch(3, \@ra);
 print Dumper fetch(6, \@ra);
 ;;
 sub fetch {
   my ($i, $ar) = @_;
   return [ $i, $ar->[$i] ] if exists $ar->[$i];
   return [ $i-1, $ar->[$i-1] ], [ $i+1, $ar->[$i+1] ];
   }
"
$VAR1 = [
          3,
          33
        ];

$VAR1 = [
          5,
          55
        ];
$VAR2 = [
          7,
          77
        ];
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