Not sure if it's better or just different, but you could do something like this:

my @nums = (1, 5, 6, 8, 15, 20);
my $max_idx = 21;
my $pad = 1;

my @hilite = @nums;
my $hilite = shift @hilite;

my $i = -1;
for my $n (@nums) {
    for my $c ($n-$pad .. $n+$pad) {
        if ($c > $i) {
            last if $c > $max_idx;
            $i = $c;
            if ($i == $hilite) {
                print "I=$i\n";
                $hilite = shift @hilite;
            } else {
                print "  i=$i\n";
            }
        }
    }
}
[download]

Output:

  i=0
I=1
  i=2
  i=4
I=5
I=6
  i=7
I=8
  i=9
  i=14
I=15
  i=16
  i=19
I=20
  i=21
[download]

Update: changed context to $pad = 1 to keep the example output in line with the other replies (Apparently it was asking too much to figure out that I had originally set $pad = 2 ... — Or why the downvotes?)