It is definitely more fun if you *don't* use Set::IntSpan...

#!/usr/bin/perl
use strict;
use warnings;
use Set::IntSpan;

my @array = ("1", "2", "3", "5", "6", "7", "9");
my $set   = Set::IntSpan->new(\@array);
my $out   = join(',', $set->D($set->holes()));

print $out, "\n";
[download]

OUTPUT:

1-3,5-7,9
[download]