#!/usr/bin/perl -w
use Fcntl qw(SEEK_END);
$usage = "usage: $0 <gigs> <file>\n";

$gigs = shift and $gigs > 0 or die $usage;
$file = shift or die $usage;

$bytes =  (2**10)**3 * $gigs;

open FILE, "+<", $file or die "opening $file: $!\n";
sysseek FILE, $bytes - 1, SEEK_END or die "sysseek: $!\n";
syswrite FILE, "\0" or die "syswrite: $!\n";
close FILE or die "closing $file: $!\n";