#!/usr/bin/perl -w
use strict;

use constant BLOCKSIZE => 128 * 1024;

die "Usage: $0 numbytes file" unless @ARGV > 1;
my $remove_bytes = shift @ARGV;
my $file         = shift @ARGV;

open(my $fh, "+<", $file) or die "Couldn't open $file: $!\n";

if(-s $fh > $remove_bytes ) {
    seek $fh, $remove_bytes, 0;
    my $total_in = 0;
    while(my $bytes_in = read $fh, my $buf, BLOCKSIZE) {
        seek $fh, $total_in, 0;
        print $fh $buf;
        last if $bytes_in < BLOCKSIZE;
        $total_in += $bytes_in;
        seek $fh, $remove_bytes + $total_in, 0;
    }
}
truncate $fh, tell $fh;