I have these 4 statements

```
$ perl -e ' $in="axaxaxhhhh"; $in=~s/a/p/ for 1..2 ; print $in '  # 1. Ok
pxpxaxhhhh

$ perl -e ' $_="axaxaxhhhh"; $_=~s/a/p/ for 1..2 ; print $_ ' # 2. Why not working?
axaxaxhhhh

$ perl -e ' $_="axaxaxhhhh"; $_=~s/a/p/g; print $_ ' # 3.good
pxpxpxhhhh

$ perl -e ' $_="axaxaxhhhh"; $_=~s/a/p/; print $_ ' # 4.good 
pxaxaxhhhh

$

```

what is happening in my second case ?. why it is not modifying $\_?