whats the difference between these two 'new' constructors in perl?

   1) sub new {
     my $class = shift;
     my $self = {};
     $self->{firstName} = undef;
     $self->{lastName} = undef;
     $self->{PEERS} = [];
     bless ($self, $class);
     return $self;
     }

  2) sub new {
        my $class = shift;
        my $self = {
           _firstName => shift,
           _lastName  => shift,
           _ssn       => shift,
        };
        bless $self, $class;
        return $self;
    }

I am using the 2nd one so far, but i need to implement the arrays in perl? can you suggest a way to do it with the 2nd 'new' constructor