#! perl -slw
use strict;
use Fcntl qw[ SEEK_CUR SEEK_SET ];
use constant BUFSIZE => 64 * 1024;

our $RECLEN || die "you must specify the length of the header. -RECLEN=nnn";
@ARGV or die "No filename";

open FILE, '+<:raw', $ARGV[ 0 ]
    or die "$!: $ARGV[ 0 ]";

sysread FILE, my $header, $RECLEN or die "sysread: $!";

my( $nextWrite, $nextRead ) = 0;
while( sysread FILE, my $buffer, BUFSIZE ) {

     $nextRead = sysseek FILE, 0, SEEK_CUR
        or die "Seek query next read failed; $!";

     sysseek FILE, $nextWrite, SEEK_SET
        or die "Seek next write failed: $!";

     syswrite FILE, $buffer
        or die "Write failed: $!";;

     $nextWrite = sysseek FILE, 0, SEEK_CUR
        or die "Seek query next write failed $!";

     sysseek FILE, $nextRead, SEEK_SET
        or die "Seek next Read failed: $!";
}

truncate FILE, $nextWrite or die "truncate failed: $!";
close FILE or die "close failed: $!";