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

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

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

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

while ( $str =~ /\G$re_start/g ) {
      print "$+{start}: "; 
      while ( $str =~ /\G$re_repeat/gc ) {
            print "$+{repeat} ";
      }
      print "\n";
}


my @res;
$str =~ /
            (?:
                $re_start
                (?{ push @res, $+{start} })

                (?:
                    $re_repeat
                    (?{ push @res, $+{repeat} })
                )*
            )*
        /gx;

dd \@res;