my ($min,$max) = (0,21);
my @nums = (1, 5, 6, 8, 15, 20);
my $pad = 1;
my %line;
map { $line{$_}++ for ($_-$pad..$_+$pad) } @nums;
my @padded = sort { $a <=> $b } keys %line;

# print the lines
for (@padded){
    if (@nums && $_ == $nums[0]){
        print "Hilighted: $_\n";
        shift @nums;
    } elsif ($min <= $_ && $_ <= $max) {
        print "$_\n";
    }
}

##</code><code>##

0
Hilighted: 1
2
4
Hilighted: 5
Hilighted: 6
7
Hilighted: 8
9
14
Hilighted: 15
16
19
Hilighted: 20
21