use strict;
use warnings;

my $re0;
my $re1;

our $s_pos;

$re1 = qr/
          (?{ local $s_pos = pos; })
          [A-Z]
          (?{ print("[", substr($_, $s_pos, pos()-$s_pos), "]\n") })
       /x;

$re0 = qr/
          (?:
             (??{ $re1 })
          )+
       /x;

'abcDE' =~ $re0;

__END__
output
======
(?:
             (??{ $re1 })
          )+
        matches null string many times before HERE mark in regex m/
          (?:
             (??{ $re1 })
          )+
        << HERE / at script.pl line 19.
[D]
[E]

##</code><code>##

use strict;
use warnings;

my $re0;
my $re1;

$re1 = qr/
          ([A-Z])
          (?{ print("[$1]\n") })
       /x;

$re0 = qr/
          (?:
             ()
             (??{ $re1 })
          )+
       /x;

'abcDE' =~ $re0;

__END__
(?:
             ()
             (??{ $re1 })
          )+
        matches null string many times before HERE mark in regex m/
          (?:
             ()
             (??{ $re1 })
          )+
        << HERE / at script.pl line 17.
[D]
[E]