As far as I can tell, cmp_deeply() does exactly what is_deeply() from Test::More does.

Actually, cmp_deeply does a lot more than is_deeply. At its most basic, it does the same thing, but at its most complex, it is soooooo much more. Here is an example from the docs:

cmp_deeply(
    $obj,
    listmethods(
      name => "John",
      ["favourites", "food"] => ["Mapo tofu", "Gongbao chicken"]
    )
  );
[download]

This is the Test::More equvalent of doing:

is($obj->name, "John", '... our name is John);
is_deeply([$obj->favourites("food")], ["Mapo tofu", "Gongbao chicken"]
+);
[download]

And it gets even better than that when you start looking at the superhash and subhash functions. I have used Test::Deep to validate a large tree-ish data-structure which is used to configure an application. This is all accomplished in a single test, here is a snippet of that code:

my $report = do($tree_file) || die "failed to load the report tree\n=>
+ $@\n";

my $is_string = re('^(\s|\w|\d)*$');
my $is_module_name = re('^(([a-zA-Z0-9_]+)\:\:([a-zA-Z0-9_]+))+$');

cmp_deeply(
    $report,
    all(
        isa("ARRAY"), 
        array_each(
            subhashof({
                report_type      => $is_string,
                report_module    => $is_module_name,
                graphing_modules => isa("HASH"),
                question_groups  => all(
                    isa("ARRAY"), 
                    array_each(
                        subhashof({
                            question_group   => $is_string,
                            report_module    => $is_module_name,
                            graphing_modules => isa("HASH"),
                            questions        => all(
                                                isa("ARRAY"),
                                                array_each(re('^\d+$')
+)
                                                ) # all
                        }) # subhashof
                    ) # array_each
                ) # all
            }) # subhashof
        ) # array_each
    ) # all
); # cmp_deeply
[download]

To do this with Test::More would not only have been more code, but would really have required a lot of bookkeeping. With Test::Deep, it took me no time at all to but this together and it is far more flexible than any version I would have coded.