my $re = qr{(\w+)(\s*\d+\s*)*};

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

while ($str =~ /$re/g) {
    say "$&: $1 $2";
}

# output:
# a 1 2 3 : a 3 
# b 4 5 6: b 6