Greetings all,
Update
After re-reading the OP I have modified the following code to only identify those elements that are not unique to only one list and are not found in all lists.
This has the benefit of not hard-coding in '2' as a comparison since this logic may need to be extended to more lists in the future.

my @a1 = qw(1 2 5 9 11 12 13);
my @a2 = qw(1 2 5 11 12 13);
my @a3 = qw(1 2 5 7 9 12 13);

my @shared = shared(\@a1, \@a2, \@a3);

sub shared
{
    my $lim = scalar(@_);
    my @lists = map{@{$_}}@_;
    return do{my %seen;
        $seen{$_}++ for(@lists);
        map{delete $seen{$_} if($seen{$_} == 1 || $seen{$_} == $lim)}k
+eys %seen;
        sort {$a <=> $b} keys %seen;
    };
}
[download]

Output is

9 11
[download]

-InjunJoel