$ perl -E '
    my $re = qr{(?<letter>\w+)(?<digit>(?:\s*\d+\s*)*)};
    my $str = "a 1 2 3 b 4 5 6";

    while ($str =~ /$re/g) {
        say "$&: $+{letter} $+{digit}";
    }
'
a 1 2 3 : a  1 2 3
b 4 5 6: b  4 5 6