If need to set or get an ACL without uplaoding a file you can use the subcomand acl, for example to make an object private:

s3m acl <s3 provider>/<bucket>/object -a private

To retrive the existing ACL:

s3m acl <s3 provider>/<bucket>/object  | xmllint --format -

notice the usas for xmlllint to pretty print the output