my $msg_marker = "\0\0MSrc";
my $tiny_read = 16 + length( $msg_marker );
while ( ! eof ) {
    $/ = \$tiny_read;
    $_ = <>;     # marker in here?
    while ( -1 == index $_, $msg_marker and ! eof ) {
        # chop the beginning if still no marker
        $_ = substr $_, $tiny_read if length > $tiny_read;
        $_ .= <>;
    }
    $_ .= <>;        # make sure to get those 16 bytes
    $/ = "\0\0";
    $_ .= <>;        # read to the end of the message

    message_in_here( $_ );
}