I am assuming here that Perl processes each array element char by char "from left to right"
You assume incorrectly. The array is processed element-by-element, not character-by-character. The first element of the array is ca
and the second element is qw
On the first pass through the loop, the flip-flop's state is false, so ca is tested against /a/, which matches, so it returns true, but it also tests against /c/ which resets it to false. From perldoc perlop:
Once the left operand is true, the range operator stays true until the right operand is true, AFTER which the range operator becomes false again. It doesn't become false till the next time the range operator is evaluated. It can test the right operand and become false on the same evaluation it became true (as in awk), but it still returns true once. If you don't want it to test the right operand until the next evaluation, as in sed, just use three dots ("...") instead of two. In all other regards, "..." behaves just like ".." does.