$_ = 'aaabbb';

while (/((?: a | c? )*)/xg) {
    printf "1: '%s' '%d'\n", $&, pos;  
}
while (/(( a )* | ( c? )?)/xg) {
    printf "2: '%s' '%d'\n", $&, pos;  
}

__END__
1: 'aaa' '3'
1: '' '3'
1: '' '4'
1: '' '5'
1: '' '6'
2: 'aaa' '3'
2: '' '3'
2: '' '4'
2: '' '5'
2: '' '6'