They're the same as the x (extended space-insensitive), i (case insensitive), s (single line, allow . to match \n.), and m (multi line, allow ^ and $ to match around \n) modifiers you can put on the end of a regex m// match.
When you save a modified regex using quote-regex (which Regex::Assemble uses internally), like this $regex = qr/foo/ix, these flags need to be set within the $regex, particularly because you might well incorporate it into a larger quoted regex at some later point, like this: $fwibble = qr/$regex|something_else/. The flags are only in operation for the first part of this new regex, but not for something_else.
The flags ahead of the dash are the ones in operation within the scope of the enclosing parentheses. So (?-xism:foo) means that none of the flags are in operation when trying to match 'foo'. (?misx:foo) would mean all of them are in operation, and (?ix-sm:foo) would mean only i and x were.