Please note: prototypes are ignored on method calls. (Sorry!)

# reworked slightly, for conciseness.
sub new
{
  my %self;
  ( my $class, $self{qw( from_node to_node label )} ) = @_;
  bless \%self, $class;
}
[download]

The point is that the subroutine can deal with the arguments itself, including the number of arguments, in any way it sees fit. If you want specifically to disallow the Edge->new("from_node"); case (not unreasonable), then you could do something like

sub new
{
  @_ == 2 and die "Error: can't set just one of the links!";
  my %self;
  ( my $class, $self{qw( from_node to_node label )} ) = @_;
  bless \%self, $class;
}
[download]

or even

sub new
{
  my %self;
  ( my $class, $self{qw( from_node to_node label )} ) = @_;
  $self{'from_node'} && ! $self{'to_node'} and
    die "Error: can't set just one of the links!";
  bless \%self, $class;
}
[download]

jdporter
The 6th Rule of Perl Club is -- There is no Rule #6.