In the example you give ^ and \G aren't helpful. However you can achieve what you want using multiple captures and alternation:

use strict;
use warnings;

my $string = " a 1 # ";

while ($string =~ /\G(?: (\s+) | ([0-9]+) | (\w+))/xgc) {
    if (defined $1) {
        print "whitespace\n";
    }
    elsif (defined $2) {
        print "integer\n";
    }
    elsif (defined $3) {
        print "word\n";
    }
}
[download]

Prints:

whitespace
word
whitespace
integer
whitespace
[download]