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]

In reply to Re^3: OO Perl: after it creates 2nd object overwrites hash from the first by FunkyMonk
in thread OO Perl: after it creates 2nd object overwrites hash from the first by flope004

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