First of all, is there a preferred style for "new" constructors? Specifically one which will be taking hash value/key pairs as attributes?

A summary of Perl Best Practices (TM) can be found on the Best Practices(TM) reference card. It does not make any pronouncements on the argument list for a constructor. The key-value syntax is a good choice, but you probably want to include a mod 2 test (die "Uneven list" if @_ % 2) to make sure the list is actually balanced. A single hash-ref is also a popular choice. See Moose::Manual for how the most standard issue object object system does it.

Secondly, is there a way to tell how many objects have been created/exist in a particular class?
You could add a counter to your constructor, and then provide an accessor to the counter, e.g. 
my $obj_count = 0;
sub new {
  my $class=shift;
  my %params=@_;
  my $self={};
  $self->{$_}=$params{$_} foreach keys(%params);
  bless($self, $class);
  $obj_count++;
  return $self;
}

sub get_count {
  return $obj_count;
}

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with the possibility of also adding a decrement to the destructor.
And lastly, what is the best way to access individual object attributes? Always through a method or can you access them directly?
This one, best practices does weigh in on. Always through a accessor. That allows for a layer of abstraction between the object itself, makes you insensitive to implementation changes, and allows for side-effects and internal mutations of the class if the designer needs them.

Side note: if this is not just a learning exercise, consider using a declarative object system, like Moose or one of its lighter-weight derivatives.

#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.

In reply to Re: OO best practice basic questions by kennethk
in thread OO best practice basic questions by Amblikai

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