use File::Slurp;

open my $in_file, '<', "file";
binmode($file);

open ( my $out_file, '>', "reversed" ); 
binmode($out_file);

my ( $buf, $data, $n, $bytes ); 

while (( $n = read $in_file, $data, 4096 ) != 0 ) {
	syswrite $out_file, pack( "v*", unpack("n*", $data ));   
	$bytes+=$n;
}