open( SEQ, ">sequential_output.file" );
$byteoffset = 0;
while there's data to be written {
   $breakpoint{$byteoffset} = "" if this location might
                   need to get an insertion at a later point
   if I have sequential data {
       print SEQ $sequential_data;
       $byte_offset += length($sequential_data);
   }
   else {  # this is data that needs to be back-fitted to
           # a byteoffset that I stored earlier
       $back_offset = whatever previous byteoffset is right
       $breakpoint{$back_offset} .= $backfit_data;
   }
}
close SEQ;
$breakpoint{$byteoffset} = "";
# do that so the following loop will handle the final chunk

# final loop: put all the pieces together
# in the intended order
open(SEQ, "<sequential_output.file");
open(OUT, ">final_output.file");
foreach $chunk ( sort {$a<=>$b} keys %breakpoint ) {
   sysread( SEQ, $chunkdata, $chunk );
   print OUT $chunkdata, $breakpoint{$chunk}
}