If you want to do something that isn't simple, don't use LWP::Simple. Use its bigger brother LWP::UserAgent. When invoking get, set the :content_cb and :read_size_hint fields, and die from the handler you set with :content_db after receiving 300 bytes.

Alternatively, forget about LWP::*. HTTP is quite simple, after the TCP handshake, it's one message from the client to the server, followed by one message from the server to the client. So, just open a socket, write your request on the socket, read back the reply, close the socket after reading 300 bytes.