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

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

      (?&y)+
   }x;
}

print("\n");

##</code><code>##

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

sub CAP {
   my ($idx) = $_[0];
   $idx += @- if $idx < 0;  # @+ isn't the same size as @-
   substr($_, $-[$idx], $+[$idx] - $-[$idx])
}

for ('abcdef') {
   m{
      (?(DEFINE)
         (?<x>
            ( . ) ( . )
            (?{ '...' })
         )
         (?<y>
            ( . ) ( . )
# (?{
#    printf('[%s,%s]',
#       ($#-),  # Prints 6, the number of preceding captures
#       ($#+),  # Prints 9, the total number of captures
#    )
# })
            (?{ printf('(%s,%s)', CAP(-2), CAP(-1) ); })
         )
         (?<z>
            ( . ) ( . )
            (?{ '...' })
         )
      )

      (?&y)+
   }x;
}

print("\n");