I'm actually proud of this one:

@numbers = (1,2,3,4,8,9,10,13,14,18,19,20,24);

for(@numbers) {
    push @$range,$_ and next
        if $range and $_-$range->[-1]==1;
    push @ranges, ($range = [$_]);
}        

$rangelist = join(',',
    map {
        @$_>2 ?
            "$_->[0]-$_->[-1]" :
            @$_
        } @ranges
    );
    
print "($rangelist)";
[download]

The for-loop builds an array of arrays, where each subarray is a sequence on consecutive numbers. The join/map turns the bigger subarrays into the '-' notation.

Happy coding!

Update: Yes, I agree, the regex is cooler... I still like mine though!