use 5.010;  # re features
use strict;
use warnings;

for ('abcdef') {
   m{
      (?(DEFINE)
         (?<x>
            ( . ) ( . )
            (?{ '...' })
         )
         (?<y>
            ( . ) ( . )
            (?{ print("(???,$^N)") })
         )
         (?<z>
            ( . ) ( . )
            (?{ '...' })
         )
      )

      (?&y)+
   }x;
}

print("\n");