How to create a directory with rex

I think the documentation in the rex book is a little older.

Please take a look here https://www.rexify.org/docs/api/ or (as of today) the most current docs here:
https://www.rexify.org/docs/api/1.4/rex/commands/file.pm.html#file-file_name-options-
(it says version 1.4 while the current cpan version is 1.6, but nevermind)

So, to answer your question with an example:

task "backuptask", group => "mygroup", sub { 
    #
    # 1.) define the Backup Dir (you could do it without this step)                                                                                                                        
    my $backupdir = "/tmp/backup";  
    #                                                                                                      
    # 2.) "ensure" that the file is a (existing) "directory"        
    file $backupdir, 
            ensure=> "directory", 
            owner => "myowner", 
            group => "mygroup", 
            mode => 700, 
            on_change => sub { say "File was changed";};

    #                                                                                                                                    
    # 3.) define Backup File                                                                                                                         
    my $currTimestamp = strftime('%y%m%d',localtime);                                                                                     
    my $backupfile = "$backupdir/somebackup$currTimestamp";
    #                                                                            
    # 4.) "ensure" the the file is "present" at the defined path
    file $backupfile, ensure=> "present";

    ...execute something here...

}  ;