El Constructor: Solución

Para simplificar la práctica aquí esta un constructor que cumple los requerimientos:

lhp@nereida:~/Lperl/src$ sed -ne '31,74p' Number/Fraction.pm | cat -n
 1  {
 2    my $isint = qr{^\s*-?\d+\s*$};
 3    my $isfrac =  qr{^\s*(-?\d+)(/(-?\d+))?$};
 4
 5    sub new {
 6      my $class = shift;
 7
 8      my $self;
 9      if (@_ >= 2) {
10        return unless $_[0] =~ $isint and $_[1] =~ $isint;
11
12        $self->{num} = $_[0];
13        unless ($_[1]) {
14          carp "Can't make a $class with demominator 0";
15          return;
16        }
17        $self->{den} = $_[1];
18      }
19      elsif (@_ == 1) {
20        if (ref $_[0]) { # Number::Fraction->new($x) y $x objeto. Lo copiamos
21          return $class->new($_[0]->{num}, $_[0]->{den}) if (UNIVERSAL::isa($_[0], __PACKAGE__));
22          croak "Can't make a $class from a ", ref $_[0];
23        } else { # Es una cadena 'num/num' o 'num'
24          return unless $_[0] =~ $isfrac;
25
26          $self->{num} = $1;
27          $self->{den} = $3 || 1;
28        }
29      }
30      elsif (ref($class)) { # LLamado $x->new. Copiamos el objeto
31        %$self = %$class;
32      }
33      else {
34        $self->{num} = 0;
35        $self->{den} = 1;
36      }
37
38      bless $self, ref($class) || $class;
39
40      $self->normalise;
41
42      return $self;
43    }
44  }