...tell me why the two can come to the same result...

#!\usr\bin\perl -w
for($_="head";s/(.)//;){
 print"now head is $_.\n";
}

#!\usr\bin\perl -w
for($_="head";s/.//;){
 print"now head is $_.\n";
}
[download]

The only difference between your (updated) two samples, is that in the first you have capturing brackets. These will cause the matched character to be copied into $1. But then you do nothing with $1, so otherwise both samples are the same:

Match a (the first) character in $_, and then delete it; print out the contents of $_; repeat until no match occurs, which will be when $_ is empty.

What difference were you expecting?

Thank you for your explainationS to me,I know the "()" has memory function, also I know that the function of ".", I enconter

#!\usr\bin\perl -w

for($_="head";s/(.)//;){
   print"now head is $_.\n";
}
[download]

in the book learning perl, I am confused why not just "." So I ask this question.

Janitored by Arunbear - added code tags, as per Monastery guidelines

As previously mentioned, in this particular case there isn't any reason to use "(.)" over purely "."

If you were going to require the value you replaced here afterwards for another purpose then it would make sense. Otherwise "(.)" has no advantages that I'm aware of.

--- Jay

All code is untested unless otherwise stated.
All opinions expressed are my own and are intended as guidance, not gospel; please treat what I say as such and as Abigail said Think for yourself.
If in doubt ask.

They give the same output because they are still technically the same.

The only difference is the parenthesis in the regex around the "any single character" ( or "." ).

The role of parenthesis here is to group parts of the regex together and to create back references ($1, $2 etc). See the perlre documentation for full details on what it does.

In this particular case it's making no difference to the operation of the regex in question.