package MyClass;

my $instances = 0;  # Class data as a lexical

sub new {
    my $class = shift;
    my $self = {
        attribute => 'foo',
    };

    bless $self, $class;

    $instances++;

    return $self;
}

sub DESTROY {
    my $self = shift;

    $instances--;
    return;
}

# Class data accessor
# Could also be a mutator, but it doesn't makes sense here.
sub instances {
    return $instances;
}

# Class data accessor/mutator
sub attribute {
    my $self = shift;

    if( @_ ) {
        $self->{attribute} = shift;
    }

    return $self->{attribute};
}