my $new = {};
tree($new, $_, $hash{$_}) for keys %hash;

sub tree {
    my ($target, $key, $val) = @_;
    my ($char, $rest) = split /\./, $key, 2;
    unless (defined $rest) {
        $target->{$char} = $val;
        return;
    }
    $target->{$char} ||= {};
    tree($target->{$char}, $rest, $val);
}