关于defined的使用

大家好，我写了一段程序，但是发现出现语法错误，请大家指点一下。谢谢！具体源码如下：
Perl code

#!/usr/bin/perl
use 5.010;
use strict;
use diagnostics;
use feature 'state';

sub greet
{
  state $name;
  if(!defined($name)) #这里判断静态变量$name是否已经定义
  {
     $name=$_;
     print "Hi ", $_, "! ", $name, " is also here ! \n";
  }
  else
  {
     print "Hi ", $_, "! ", $name, " is also here ! \n";
  }
}

greet("Fred");
greet("Barney");

系统提示：
Use of uninitialized value $_ in print at ./Ex4_4 line 13 (#1)
  (W uninitialized) An undefined value was used as if it were already
  defined. It was interpreted as a "" or a 0, but maybe it was a mistake.
  To suppress this warning assign a defined value to your variables.

请问这个原因是什么？

你把 @_ 和 $_ 弄混了 
Perl code

  state $name;
  $_=shift; #加上这行就可以了
  if(!defined($name))

1楼的朋友，加了还是不行啊，还是有问题。