#! /usr/bin/perl

use strict;
use warnings;

my $file = do {local $/, <DATA>};

my @cvs = $file =~ /((?:(?:[^\n@]+|@[^@]*@)\n?)+)/gs;
print "matched [[$_]]\n" for @cvs;

__DATA__

line 1
line 2
line 3
longer @line@ with @some stuff@
line with nothing on it

line 4