HTTP::Session versions before 0.54 for Perl allow memcached command injection via unfiltered session ids in HTTP::Session::Store::CHI.
The session id from the client's cookie reaches the CHI cache unchanged. The sibling HTTP::Session::Store::Memcached rejects ids that contain control characters, but the CHI store has no such check, and CHI does not escape keys for its memcached drivers.
With a memcached driver, a CR and LF in the session id end the cache command early, and the server reads the bytes after them as further commands.
Upgrade to http-session 0.54 or later.