See the current Perl documentation for msgsnd.
Here is our local, out-dated (pre-5.6) version:

msgsnd - send a SysV IPC message to a message queue

msgsnd ID,MSG,FLAGS

Calls the System
V
IPC function msgsnd to send the message
MSG to the message queue
ID.
MSG must begin with the long integer message type, which may be created with
pack("l", $type). Returns
TRUE if successful, or
FALSE if there is an error. See also
IPC::SysV
and IPC::SysV::Msg documentation.