In the interest of using only one iteration to produce the desired results, I rewrote my solution as follows. Note the use of two hashes: one to check for highlighted-ness, and one to check for already-printed-ness.

#!/usr/bin/perl
use strict;

my ($min,$max) = (0,21);
my @nums = (1, 5, 6, 8, 15, 20);
my $pad = 1;

my (%printed,%nums);
map { $nums{$_}++ } @nums;
for my $i (@nums) {
  for my $n ($i-$pad .. $i+$pad) {
    if ( ! $printed{$n} && $min <= $n && $n <= $max ) {
      print exists $nums{$n} ? "Highlighted: " : "";
      print $n, "\n";
      $printed{$n}++;
    }
  }
}
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Prints the same as before, but unfortunately it has to iterate once to build %nums and again to do the rest. I've tried several ways, but couldn't get shorter than two iterations. It is, however, of O(N) instead of O(N^2). I also believe this solution is easier to follow than my last one.