The addition of the extra braces has created a while loop that loops through the file(s), a block with an assignment and a block with an empty statement and a continue. In effect it has disassociated the continue from the while.
The typeglob assignment *_ = *. has the effect, among other things, of setting $_ = $.. Since the while has already looped through the file(s) $. is now the number of lines in the file(s).
The last action of the program will be to enter the continue and print $_ so that the number of lines is output. The -l command line option helpfully appends a newline.