sub foo () {
	my %hash = @_;
	my @array = ( sort {$a <=> $b}  (keys %hash) );
	my $j = shift @array;
	my ($periods, $i);
	my $string = "($j";

	foreach $i (@array) {
		if ($i == $j + 1) {
			$periods = 1;
		} 
		else {
			if ($periods) {
				$string .= "..$j, $i";
			}
			else {
				$string .= ", $i";		
			}
			$periods = 0;
		}
		$j = $i;
	}
	if ($periods) { 
		$string .= "..$j)"
		} 
	else { 
		$string .= ")";
	}
	return $string;
}

# a test
my @test_nums = (0, 10, 11, 12, 2, 4, 3, 5, 6, 7, 9);
my %test;

# pass via hash, only because I ultimately need to
foreach my $num (@test_nums) { 
	$test{$num} = undef;
}

#test
print &foo(%test);