use Sys::Mmap;
use constant BDfname=>"bigdatafile";
use constant BD_RECORDSIZE=>9876;
-s BDfname % BD_RECORDSIZE and die"size ne mult of recsize";
open BIGDATA, BDfname or die $!;
mmap(
    $BD,
    -s BDfname,
    PROT_READ,
    MAP_SHARED,
    BIGDATA
);
sub BDrec($){
   substr($BD,BD_RECORDSIZE*$_[0],BD_RECORDSIZE)
}