#!/usr/bin/perl

open my $out, '>:raw', '/path/to/data.offsets'
    or die $!;
open my $in, '<', '/path/to/data.dat'
    or die $!;
my $offset = 0;

local ($_,$\);
my ($this, $last) = 0; 
while (<$in>) {
    ($last, $this) = ($this, tell $in);
    print $out pack 'i', $last;
}

close $in or warn $!;
close $out or warn $!;