If there is nothing to read,it means the sysread has finished all the reading?

Or an error. You got error EAGAIN. That error signifies that sysread should block, but the file handle is non-blocking.

EAGAIN or EWOULDBLOCK: The file descriptor fd refers to a socket and has been marked nonblocking (O_NON-BLOCK), and the write would block. POSIX.1-2001 allows either error to be returned for this case, and does not require these constants to have the same value, so a portable application should check for both possibilities.

What you want:

my $buf;

while (1) {
   my $rv = sysread($out, $buf, 64*1024, length($buf));
   if (!defined($rv) && !$!{EAGAIN} && !$!{EWOULDBLOCK}) {
       die $!;
   }

   if (defined($rv) && !$rv) {
       last;
   }
   
   ... whatever it is you wanted to do instead of blocking ...
}

print $buf;
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