You have to check the return value of sysread, like so

'memory read' => sub {
    sysread( MEMORY, $data, 0, $size )
      // die $!;
}
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you'll see 'Bad file descriptor'. sysread cannot read from a filehandle opened on a string; it needs a real file descriptor. Also, you probably meant sysread( MEMORY, $data, $size, 0 ) (the third argument is the number of bytes to read and the fourth is offset - thats different from shmread).

shmread will probably still be slower, since it reads or writes the System V shared memory segment... by attaching to it, copying in/out, and detaching from it.