What can make you to understand better is to know the meaning of the code {}.

{} means a reference to an anonoymous HASH. Is the same thing of the code above, but without give a name to the hash:

  # this...
  my $self = {} ;
  # ... is the same of:
  my %foo ;
  my $self = \%foo ;
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Other thing that you should know is that each time that you make \%foo or {} we will point to a new HASH. So, each time that we go inside the sub new(), a new %foo will be created (since %foo is local due my). Or each time that we execute {} we have a reference to a new anonymous HASH. We can see that above:

  my @hold_ref ;
  for(1..3) {
    my $hash_ref = {} ;
    push(@hold_ref , $hash_ref) ;
    print "$hash_ref\n" ;
  }
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Output:

HASH(0x1a7f10c)
HASH(0x1a7f130)
HASH(0x1a75468)
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Note that the array @hold_ref is just to ensure that we won't have new HASHES created in the same address of old hashes already cleanned from the memory.

So, bless() will always bless a new reference, what means that we have a new object (well this is why we call by the name new() or create()). The meaning of bless is to say that some reference belong to a package/class, and with that we can do a lot of things, and one of them is to call a method, that is the basic of Object Orientation.

Other question is "Why Perl does OO with a reference to some data structure that belongs to a package?". Well, the main purpose of OO is to have a data structure related to the methods that will interact/change this data. Well, actually OO borned in this way, but they forgot to tell this to us.