c:\test>type indexFile.pl
#! perl -sw
use strict;

open INDEX, '>:raw', "$ARGV[ 0 ].idx" or die $!;
syswrite INDEX, pack( 'N', 0 ), 4;
syswrite INDEX, pack( 'N', tell *ARGV ), 4 while <>;
close INDEX;


c:\test>indexFile junk.dat

c:\test>dir junk.dat*
21/12/2011  17:45               425 junk.dat
21/12/2011  17:46               104 junk.dat.idx
c:\test>