...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";
}
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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?