编译器抱怨它“无法在Parser.pm第16行修改非左值子程序调用”.

我不太明白为什么我不能只分配一个新对象.我想用Moose做一个更好或更有效的方法来做可选参数？

#!/usr/bin/perl -w

package Parser;

use Moose;

require URLSpan;

require WWW::Mechanize;

has 'urlspan' => (is => 'rw',isa => 'URLSpan',required => 1);
has 'mech' => (is => 'rw',isa => 'WWW::Mechanize');

sub BUILD {
    my $self = shift;
    if(!$self->mech) {
        warn("no Mech set for " . $self->urlspan->name);
        $self->mech = WWW::Mechanize->new(agent => 'Mozilla/5.0 (Windows; U; Windows NT 6.0; de; rv:1.9.0.6) Gecko/2009011913 Firefox/3.0.4',stack_depth => 1
                                         ); #line 16
        }

}