You're not making any sense. Line by line reading (while (<>)) is buffered. sysread, on the other hand, provides no buffering.

If you want to provide your own buffering instead of using Perl's, you could do

my $buf = '';
for (;;) {
   my $rv = sysread($fh, $buf, BLOCK_SIZE, length($buf));
   die("sysread: $!") if !defined($rv);
   last if !$rv;

   process_line($1) while s/^([^\n]*\n)//;
}

process_line($buf) if length($buf);
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Update: Fixed problem mentioned by ibm1620 in comment.

Please check that regular expression again :) (to be exact, the substitution expression.)