undef

If it is undefined , we return undef as well .

如果此变量没有被定义，将返回undef。

Replace the # undef macros with # define and insert the version information .

把undef宏替换为define并插入版本信息。

Setting $ / to undef has the effect of reading all the input into $ text at once , newlines included .

将$/设置为undef的作用是将所有输入（包括换行）立刻读入$text中。

I should mention that this uses the CPAN Every module to insert either nothing ( an undef value ) or the current loop iteration in the @ labels list .

应该指出，这里使用了CPANEvery模块来插入空的内容（一个undef值）或@labels列表的当前循环迭代。

If the document_root doesn 't have any child nodes , for example , config_node will be set to undef , and the third line of the example will raise an error .

例如，如果documentroot没有任何子节点，则将confignode设置成undef，并且该示例的第三行将产生一个错误。