c:\@Work\Perl\monks>perl
use strict;
use warnings;

my %ids;

sub intersect {

    my %count;
    my @ret;
    my $count = keys %ids;
    foreach my $key (keys %ids) {
        my %this;
        for (@{$ids{$key}}) {
            next if $this{$_};
            $count{$_}++;
            $this{$_}++;
            if ($count{$_} == $count) {
                push @ret, $_;
            }
        }
    }

    undef %ids;
    return @ret;
}

$ids{'foo'} = [ qw(three four four five) ];
$ids{'bar'} = [ qw(three) ];
$ids{'zoo'} = [ qw(five four three) ];

print "[ ",join(', ',intersect)," ]\n";

__END__
[ three ]
[download]