That's not very complicated:
Well, maybe, but you are not asking the OP. That is: "I need a regular expression to match the following pattern.". You are using 2. (and perliff above uses 3!).
Update: Solving the problem with 2+ regexps is more or less trivial, but using just 1 is far more interesting. Here is a solution using look-ahead assertions (see perlre):
my $re = qr{ ^ # match at the beginning.
(?! # Look-ahead assertion: Fail if the next matc
+hes
.* first # match "first" everywhere except at the begi
+nning of the string
) # end of look-ahead assertion
This .* \. # Match "This" (at the beginning), followed b
+y anything and a dot
$ # Match the end of a string.
}x; # Allow these comments
