$a = "ab cdef  gh  ijklmn";
$b = "xyz";
while($a =~ /\G(\w\w)/g) {
    print "1: match in \$a: $1\n";
    if($b =~ /\G(\w)/g) {
        print "2: match in \$b: $1\n";
    } else {
        print "2: no match in \$b - replacing \$a\n";
        $a = "AB CD EF";
    }
    if($a =~ /\G(\s*)/g && length $1) { # The match itself never fails
        print "3: skipping whitespace in \$a\n";
    }
}