Hack using (?{ code }), which is not experimental any more at least as of 5.22(.0+) judging from the docs, and was available at least as of 5.8(.8+), although I don't know how stable it has been in time:

#!/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'
#         ];
[download]

Update: as noted below in a comment, the capture BAR is not needed. I added it while playing with %- and %+, and forgot to remove it before posting the example... I'm leaving it though, so that the comment below can still make sense. Anyway, always remember the old adage about regular expressions...