I'd have to say that it was not what I expected. I'm also surprised that this hasn't bitten me before. While I know theoretically that * can match empty strings, perl has always seemed to me to do the intuitive thing.

I guess I generally don't replace nothing with something. Usually when I use * I'm either just skipping over white space (or the moral equivalent), or I replace any pattern than has * in it with itself. That is, something like  s/A(.*)B/C${1}D/g;

Generally, I also try to constrain my patterns more, so I usually avoid constructs like ".*" or "*?", and would write something like s/A([^B]*)B/C${1}D/g;

The /s doesn't seem to have anything to do with it, except that, of course, you included a \n in your string. For example,

my $string = "aaab";
$string =~ s/a*/go/g;
[download]

Now string is "gogobgo".