The answers to your questions are in Regexp Quote Like Operators but you might find perlretut a gentler introduction. You should take the time to read and become familiar with both of these and also with prelre.

If the "/g" option is not used, "m//" in list context returns a list consisting of the subexpressions matched by the parentheses in the pattern, i.e., ($1, $2, $3...). (Note that here $1 etc. are also set, and that this differs from Perl 4’s behavior.) When there are no parentheses in the pattern, the return value is the list "(1)" for success. With or without parentheses, an empty list is returned upon failure.

In your first two cases there are no parentheses and the pattern matches so the list "(1)" is returned. In the last two cases there is a pair of parentheses so the match returns a list consisting of what matched the enclosed pattern: a single character in the third case and a sequence of word characters in the fourth.