print FILE chr(78);

$number = ord('N');
[download]

ok that is fine if ASCII only covers 0-127. The file I write to can be a binary file rather than ascii, so will the same functions work in that case?

Yes, they will work. Perl uses Latin1 in the range 0..255.

print FILE pack "C*", @numbers; # write
@numbers = unpack "C*", <FILE>; # read
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_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

Binary files aren't likely to be LF-terminated, so <...> isn't likely to be appropriate.

sub read_block {
   my ($fh, $bytes_to_read) = @_;
   my $buf = '';
   while ($bytes_to_read) {
      my $bytes_read = read($fh, $buf, $bytes_to_read, length($buf));
      die("Unable to read from file: $!\n")
         if !defined($bytes_read);
      die("Unable to read from file: Unexpected end of file\n")
         if !$bytes_read;
      $bytes_to_read -= $bytes_read;
   }
   return $buf;
}

# Write
print $fh (pack('C',  scalar(@numbers));
print $fh (pack('C*', @numbers));

# Read
my $num_numbers = unpack('C',  read_block($fh, 1));
my @numbers     = unpack('C*', read_block($fh, $num_numbers));
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Update: Fixed incorrect variable name. Thanks blokhead.

Binary files aren't likely to be LF-terminated, so <...> isn't likely to be appropriate.

As always, that depends. It is likely to be appropriate if the file isn't too big and read in slurp mode.

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}

print FILE pack('C', 78);
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