The regular expression:

(?s-imx:((?:(?:[^\n@]+|@[^@]*@)\n?)+))

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?s-imx:                 group, but do not capture (with . matching
                         \n) (case-sensitive) (with ^ and $ matching
                         normally) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    (?:                      group, but do not capture (1 or more
                             times (matching the most amount
                             possible)):
----------------------------------------------------------------------
      (?:                      group, but do not capture:
----------------------------------------------------------------------
        [^\n@]+                  any character except: '\n'
                                 (newline), '@' (1 or more times
                                 (matching the most amount possible))
----------------------------------------------------------------------
       |                        OR
----------------------------------------------------------------------
        @                        '@'
----------------------------------------------------------------------
        [^@]*                    any character except: '@' (0 or more
                                 times (matching the most amount
                                 possible))
----------------------------------------------------------------------
        @                        '@'
----------------------------------------------------------------------
      )                        end of grouping
----------------------------------------------------------------------
      \n?                      '\n' (newline) (optional (matching the
                               most amount possible))
----------------------------------------------------------------------
    )+                       end of grouping
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

##</code><code>##

use warnings;
use strict;

use YAPE::Regex::Explain;

my $REx = qr/((?:(?:[^\n@]+|@[^@]*@)\n?)+)/s;

print YAPE::Regex::Explain->new($REx)->explain;


##</code><code>##

my $REx = 
    qr/
          (                        
              (?:                  
                  (?:              
                        [^\n@]+  
                      |            
                        @
                        [^@]*
                        @  
                  )                
                  \n?              
              )+                   
          )                        
      /gsx;