What do you get if you try this on the file?
my $fname = "Words.txt";
my $fsize = -s $fname;
open( my $fh, '<', $fname ) or die "$fname: $!\n";
$/ = undef;
my $data = <$fh>;
my $length = length( $data );
my $nnulls = tr/\x00/ /;
print "Read $length of $fsize bytes, changed $nnulls nulls to spaces\n
That will just report the basic numbers to see whether you've read the entire file, and how many null bytes it contains.
As for the stuff you're doing with ord(substr(...)) + 1; ... that looks like the sort of thing that should be done when you still have null bytes in the file (i.e. without converting the nulls to spaces). And it looks like the sort of thing you should be doing with unpack, as others have mentioned.