I follow the doc to learn perl .

Are you reading "Perl 5 Tutorial" by Chan Bernard Ki Hong? (http://www.cbkihong.com/index.pl?op=perltut&lang=e) Be aware that it is 11 years old now, and some of the code examples are showing their age, such as not using strict or the three-arg open. (Plus I imagine having to change all those single quotes and removing line numbers will get a little annoying.)

There are lots of other places to learn Perl, lots of books including Learning Perl, or the resources on this site.