use v5.12;
use warnings;
use Data::Dump;

my $str = 'a 1 2 3 b 4 5 6';

my $re_start  = qr{(?<start>\w+)\s*};
my $re_repeat = qr{\s*(?<repeat>\d+)\s*};


# --- approach 1 with \G and /gc 
my @res1;
while ( $str =~ /\G$re_start/g ) {
      push @res1, [$+{start}]; 
      while ($str =~ /\G$re_repeat/gc) {
            push @{ $res1[-1] }, $+{repeat};
      }
}
dd \@res1;

# --- approach 2 with embedded (?{CODE})
my @res2;
$str =~ /
            (?:
                $re_start
                (?{ push @res2, [$+{start}] })

                (?:
                    $re_repeat
                    (?{ push @{ $res2[-1] }, $+{repeat} })
                )*
            )*
        /gx;
dd \@res2;