Your version doesn't create a new hashref for each object. It just reuses the same one:

my $self1 = {};
%{ $self1->{hash} } = ();
print "$self1->{hash}\n";
%{ $self1->{hash} } = ();
print "$self1->{hash}\n";
#Output:
HASH(0x504f80)
HASH(0x504f80)
[download]

My use of {} creates a new hashref for both objects:

my $self2;
$self2->{hash} = {};
print "$self2->{hash}\n";
$self2->{hash} = {};
print "$self2->{hash}\n";
#Output:
HASH(0x622bb0)
HASH(0x504160)
[download]