package MyPackage;

our $global=0;

sub global {
  shift;
  $global=$_[0] if @_;
  $global;
}

sub instance {
  my $this=shift;
  if (@_) {
    $this->{instance}=$_[0];
    ++$global;
  }
  $instance;
}

sub new {
  my $package=shift;
  bless {},$package;
}

}

package main;

my $package = new MyPackage();

#main *doesn't* know what vars are per-class or which are per-instance.
# that's an implementation detail the caller doesn't know about
# so all calls are called through method calls.

$package->instance(1.7);
print "vars created = ",$package->global;
...
# reset MyPackage counter to globally reset counter for all users
# (not normal used; be available for better user control of counting...
$package->global(0);
...