use constant BLK_SIZE => 16*1024;

sub pseek {
   my( $p, $to_read, $blk_size ) = @_;
   $blk_size ||= BLK_SIZE;
   while ( $to_read ) {
      $blk_size = $to_read if $to_read < $blk_size;
      my $read = read( $p, my $discard, $blk_size );
      return $read if !$read;
      $to_read -= $read;
   }
   return 1;
}

##</code><code>##

$ perl -e'$|=1; print "a"; sleep(10); print "b"' | perl -le'read(STDIN, $buf, 10); print $buf'
ab
$ perl -e'$|=1; print "a"; sleep(10); print "b"' | perl -le'sysread(STDIN, $buf, 10); print $buf'
a