# untested, supports the interface you describe
sub commonkeys {
        my $number_of_hashes = scalar(@_);

        my %union;
        my @result;
        foreach (@_) {
                foreach (keys %{$_}) {
                        $union{$_}++;
                }
        }

        while (my ($key,$count) = each %union) {
                if ($count == $number_of_hashes) {
                        # a key which has appeared the same number of times as we have hashes, is present in every hash.
                        push(@result,$key);
                }
        }

        return @result;
}