Overloading in Perl / Number::Fraction

Number::Fraction::add

• sub add {
    my ($self, $delta) = @_;
  
    if (ref $delta) {
      if (UNIVERSAL::isa($delta, 
                         ref $self)) {
        $self->{num} = $self->{num} 
                     * $delta->{den} 
                     + $delta->{num} 
                     * $self->{den};
        $self->{den} = $self->{den} 
                     * $delta->{den};
      } else {
        croak "Can't add a ", 
              ref $l, " to a ", ref $l;
      }
    } else {
      if ($delta =~ m|(\d+)/(\d+)|) {
        $self->add(Number::Fraction->new($1, $2));
      } elsif ($delta !~ /\D/) {
        $self->add(Number::Fraction->new($delta, 1));
      } else {
        croak "Can't add $delta to a ", ref $1;
      }
    }
    $self->normalise;
  }