I like the regex solution linked elsewhere, but how about something a little bit functional?

#!/usr/bin/perl

use strict;
use warnings;

use Test::More tests => 4;

sub range
{
    my @numbers = sort { $a <=> $b } @_;

    my ($in_range, @range);

    while (@numbers > 1)
    {
        my $current = shift @numbers;
        my $gap     = $numbers[0] - $current;

        for my $action
        (
            [ sub {   $_[0] && $_[1]  > 1 }, [ ', ', 0 ] ],
            [ sub { ! $_[0] && $_[1] == 1 }, [ '..', 1 ] ],
            [ sub { ! $_[0]               }, [ ', ', 0 ] ],
        )
        {
            my ($condition, $values) = @$action;
            next unless $condition->( $in_range, $gap );

            push @range, $current,     $values->[0];
            $in_range                = $values->[1];
            last;
        }
    }

    return join('', @range, @numbers);
}

my $result = range( 0, 10, 11, 12, 2, 4, 3, 5, 6, 7, 9 );

like( $result, qr/^0/,           'range() should sort results' );
like( $result, qr/12$/,          '... including end'           );
like( $result, qr/2\.\./,        '... finding start of range'  );
is(   $result, '0, 2..7, 9..12', '... and rangifying numbers'  );
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