$| = 1;

 while (1)
 {
     $_ = <>;
     open FILE, ">/tmp/moo1"; print FILE $_; close FILE;
     chomp;
     open FILE, ">/tmp/moo2"; print FILE $_; close FILE;
     last if $_ eq "y";
 }

Checking the files:

> od -c /tmp/moo1
0000000
> od -c /tmp/moo2
0000000   y  \r
0000002