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";
            }
        }
    }
}