I'd construct the @padded list in one go by combining the map with a grep each for the tasks of keeping things within range and removing duplicates. I'd also use a hash to keep track of which lines to highlight.

use strict;
use warnings;

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

my %highlight;
@highlight{@nums} = (1) x @nums;

my @padded;
{
    my %seen;
    @padded =
       grep { ! $seen{$_} ++ }
       grep { $_ >= 1 && $_ <= $max }
       map  { $_ - $pad .. $_ + $pad }
       @nums;
}

print
   $highlight{$_}
   ? qq{Highlighted: $_\n}
   : qq{$_\n}
   for @padded;
[download]

Produces

Highlighted: 1
2
4
Highlighted: 5
Highlighted: 6
7
Highlighted: 8
9
14
Highlighted: 15
16
19
Highlighted: 20
21
[download]

I hope this is of use.

Cheers,

JohnGG