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

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