All the data ends up in processeddata, no matter how much is read by a given call to sysread.

Oops, my bad. Actually, hm, I'm not sure what happens without the offset adjustments that I have in my code...

It doesn't look like you're handling ... other exceptional conditions

He does this too. Relevant code:

      if( ! defined $length || $length == 0 ) {
        $err .= "Error from child: $!\n" unless defined $length;
        $select->remove($fh);
      }
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That treats things like EINTR as "fatal" errors (stop listening on that $fh), which is probably not what he wants.

You removed select, which is cruicial here.

I was just commenting on the vagaries of sysread().