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if ( /(BA[ABC]{2})|(CA[CD]{2})|(DA[SC]{2})/ )
   my $first =
...
     : defined( $2 ) ? 2
     : 3
     );

local our $first;

...
   |   CA[CD]{2}  (??{ $first = 2; })
   |   DA[SC]{2}  (??{ $first = 3; })
/x;