Turn an array of integers into a string of an array with ranges

DeparseRange.pl

use strict;
use warnings;
use List::Util qw(reduce);

my @in = (1,2,3,4,5,6,8,9,10,12,16,17,18,21);

print join (',',

	map { defined $_->[1] ? "$_->[0]..$_->[-1]": $_->[0] }
	@{
		reduce{ 
			my $c = [$b];
			if (@$a and ref $a->[0]){
				if ($a->[-1]->[-1] + 1 == $b->[0]){
					push @{$a->[-1]}, $b->[0];
					$c = $a;
				}
				else {
					push @$a, $b;
					$c = $a;
				}
			}
			elsif (!ref $a->[0])
			{
				$c = [$a, $b];
			}
			$c;
		}
		map { [ $_ ] }
		@in
	}

);