use strict;
use warnings;

my $re;

$re = qr/
         (\d)
         (?{ print("[$1]\n") })
       /x;

'thr33' =~ $re;

__END__
output
======
[3]

##</code><code>##

use strict;
use warnings;

my $re0;
my $re1;

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

$re0 = qr/
          (??{ $re1 })

       /x;

'thr33' =~ $re0;

__END__
output
======
Use of uninitialized value in concatenation (.) or string at (re_eval 1) line 1.
[]

##</code><code>##

use strict;
use warnings;

my $re0;
my $re1;

$re1 = qr/
          (\d)
          (?{ print("[$-[0]]\n") })
       /x;

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

'thr33' =~ $re0;

__END__
output
======
[3]
[3]

expected
========
[3]
[4]