use List::MoreUtils qw(all);
sub series {
    _series(2, @_);
}

sub _series {
    my $recursion_level = shift;
    return if $recursion_level < 0;
    return if @_ < 2;
    my $first = $_[0];
    if (all { $_ == $first } @_) {
        return $first;
    }

    my @a = map { $_[$_+1] - $_[$_] } 0 .. (@_ - 2);
    my $r = _series($recursion_level - 1, @a);
    return $_[-1] + $r if defined $r;

    # catch division by zero
    $r = eval {
        @a = map { $_[$_+1] / $_[$_] } 0 ... (@_ - 2);
        _series($recursion_level - 1, @a);
    };
    return $_[-1] * $r if defined $r;

    return;
}