First, there is an error in your example. You are creating $HoH but are dumping %HoH.

Besides that what is happening is that the code brought in by require is in it's own scope. So the variables declared with my go a way when you leave that scope. This is what our is for. If you are using an older (pre 5.6) version of perl you can use vars

$ cat HoH.pl 
use strict;
use Data::Dumper;
our $HoH;
require 'hash';
print Dumper($HoH);
[download]

or

use strict;
use Data::Dumper;
use vars qw/ $HoH /;
require 'hash';
print Dumper($HoH);
[download]

Either way you will just have to drop the my in hash (or also declare it our if you are using the first example.)