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

use constant BLOCKSIZE => 128 * 1024;

my $USAGE =<<END_OF_USAGE;
Usage: $0 nbytes file [file ...]
    nbytes    Positive integer. Number of bytes to remove.
    file      Name(s) of file(s) to decapitate.
END_OF_USAGE

die $USAGE if @ARGV < 2;

my $nbytes = shift @ARGV;
die $USAGE unless $nbytes eq int(0+$nbytes) and $nbytes >= 0;

my $completed = 0;

for my $file (@ARGV) {
    open(my $fh, "+<", $file) or warn("Couldn't open $file: $!\n"),next;
    if(-s $fh > $nbytes ) {
        seek $fh, $nbytes, 0;
        my $total_in = 0;
        while(my $bytes_in = read $fh, my $buf, BLOCKSIZE) {
            my $eof = eof($fh);
            seek $fh, $total_in, 0;
            print $fh $buf;
            $total_in += $bytes_in;
            last if $eof;
            seek $fh, $nbytes + $total_in, 0;
        }
    }
    truncate $fh, tell $fh;
    close $fh;
    $completed++;
}

exit ($completed == 0);