Perl's a great language for special variables - variables that are set up without the programmer having to intervene and providing information ranging from the number of lines read from the current input file ($.) through the current process ID ($$) and the operating system ($^O). Other special variables effect how certain operations are performed ($| controlling output buffering / flushing, for example), or are fundamental in the operation of certain facilities - no more so than $_ and @_.

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1.@_含义

1)是perl中默认的数组变量

比如说你想移除数组中的一个元素赋值给一个变$value

方法1：你可以定义某个数组如@abcd
       my $value=shift @abcd;

方法2：你没有定义任何数组
       my $value=shift @_;   和上例等效
       这里perl会隐式的选择@_

2）是sub子函数中的默认参数列表.

例如：
sub funct($$) {
    ($param1, $param2) = @_;
    #Statement
}

再例如，有下面一段代码：
my $max_number = &max(1,2);
print "1 and 2 ,the max number is $max_number\n";
sub max{
     my ($num1,$num2) = @_[0,1]; ## 取出参数列表中的元素。
     ........此处省略求max运算
}

在子函数中直接shift; 就可以从@_的前端弹出一个元素.

shift;
等于
shift @_;

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@_ is the list of incoming parameters to a sub. So if you write a sub, you refer to the first parameter in it as $_[0], the second parameter as $_[1] and so on. And you can refer to $#_ as the index number of the last parameter:
sub demo {
  print "Called with ",$#_+1," params\n";
  print "First param was $_[0]\n";

Note that the English module adds in the ability to refer to the special variables by other longer, but easier to remember, names such as @ARG for @_ and $PID for $$. But use English; can have a detrimental performance effect if you're matching regular expressions against long incoming strings

2.$_含义

1)$_为默认列表变量。在一个命令没有任何参数的时候，表示它从默认变量里读取。
例如：
print;
等于
print $_;
2)默认模式匹配空间(pattern matching space)
s/.../.../;
等于
$_ =~ s/.../.../;

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Then any regular expression matches, chops (and lcs and many more) without a parameter, and even prints assume you want to work on $_. Thus:
while ($line = <FH>) {
  if ($line =~ /Perl/) {
    print FHO $line;
    }
  print uc $line;
  }

Shortens to:
while (<FH>) {
  /Perl/ and
    print FHO ;
  print uc;
  }
3.$1,$2,...等含义

以数字为名的变量保存的是上一次匹配操作（/pattern/）中，第n个小括号中的原符号所匹配内容。  
$1就是第一对小括号中的原符号所对应的匹配内容。  
$2就是第二对小括号中的原符号所对应的匹配内容。

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