use strict;
use warnings;

my $re0;
my $re1;

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

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

'thr3.3' =~ $re0;

__END__
output
======
[3]
[5]