perl -s -nle"BEGIN{open BIG};($undef,$l)=split;read BIG,$data,$l;print $data" -- 
-BIG=bigfile.dat index.dat > outfile.dat

##</code><code>##

perl -sple"BEGIN{open BIG};($undef,$l)=split;read(BIG,$_,$l)" -- 
-BIG=bigfile.dat index.dat >outfile.dat