You never need to put braces around the hash name unless its name is wacky, and I can't figure out how to legally make one so wacky, so let's just ignore that possibility.

You need braces around a hash reference any time it is not a simple scalar variable and you are not fetching a single element, because the precedence of the dereferencing operators will not DWYM otherwise. You may put braces around a hashref any time you feel like it. And those braces around the hashref are a true BLOCK: you can put any code you like in there, as long as the final result is a hash ref. It's just like a do-block, but the do is implied by the dereferencing.

To illustrate the precedence considerations:

my $foo = {a => 1}; # $foo is a hashref
print $$foo{a}; # Ok
print @$foo{a}; # one-element slice, but ok
print $foo->{a}; # Preferred single-element fetch

my %bar = (a => {b => 1}); # %bar is a HoH
print ${$bar{a}}{b}; # Ok
print @{$bar{a}}{b}; # one-element slice, but ok
print @$bar{a}{b};   ### syntax error!
print $bar{a}{b};    # Preferred single-element fetch
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The syntax error line thinks $bar must be a reference, like $foo was earlier. But it is $bar{a} that is the reference we want to dereference.

See perldoc perlreftut for a nice statement of the (only 2!) rules for using references.