sub put {
    die "Usage: put \\hash, key1, ..., keyn, value\n" unless @_ >= 3;
    my $hash  = shift;
    die "Not a HASH reference\n" unless ref($hash) eq "HASH";
    my $value = pop;
    my $key   = pop;

    while (@_) {
        my $key        = shift;
        if (!exists($hash->{$key})) {
            $hash = $hash->{$key} = { };
            next;
        }
        elsif (ref($hash->{$key}) eq "HASH") {
            $hash = $hash->{$key};
            next;
        }
        else {
            die "Not a HASH reference\n";
        }
    }

    $hash->{$key} = $value;
}