#!/usr/bin/env perl
use strict;
use warnings;
use English qw( -no_match_vars );
use Data::Dumper;

my $string = 'foobarbaaaarbaz';
my @second;
if (my @matches = $string =~ m{\A
      (fo*)
      (?: (?<BAR>\s*ba+r) (?{push @second, $^N}))+
      (\s*ba*z) \z}mxs
   ) {
   $matches[1] = \@second;
   print Dumper \@matches;
}
# $VAR1 = [
#           'foo',
#           [
#             'bar',
#             'baaaar'
#           ],
#           'baz'
#         ];