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