use strict;
use warnings;

use Carp qw( croak );

sub avoid_utf8_internally {
   my ($s) = @_;
   utf8::downgrade($s, 1)
      or croak("Non-bytes found in input");
   return $s;
}

sub use_utf8_internally {
   my ($s) = @_;
   utf8::upgrade($s);
   return $s;
}

my $file_num;

for my $s (
   avoid_utf8_internally("bj\x{f6}rk"),
   use_utf8_internally("bj\x{f6}rk"),
   "b" x 1000,
) {
   my $packed = pack("V/a*", $s);
   printf("%s -> %s\n", length($s), length($packed));
   open(my $fh, '>', 'packed'.++$file_num) or die;
   binmode $fh;  # No crlf mucking.
   print $fh $packed;
}

##</code><code>##

>perl script.pl
5 -> 9
5 -> 9
1000 -> 1004

>debug packed1
-rcx
CX 0009
:
-d100 l9
0B14:0100  05 00 00 00 62 6A F6 72-6B                        ....bj.rk
-q

>debug packed2
-rcx
CX 0009
:
-d100 l9
0B14:0100  05 00 00 00 62 6A F6 72-6B                        ....bj.rk
-q

>debug packed3
-rcx
CX 03EC
:
-d100 3EC
0B14:0100  E8 03 00 00 62 62 62 62-62 62 62 62 62 62 62 62   ....bbbbbbbbbbbb
0B14:0110  62 62 62 62 62 62 62 62-62 62 62 62 62 62 62 62   bbbbbbbbbbbbbbbb
...
0B14:04D0  62 62 62 62 62 62 62 62-62 62 62 62 62 62 62 62   bbbbbbbbbbbbbbbb
0B14:04E0  62 62 62 62 62 62 62 62-62 62 62 62               bbbbbbbbbbbb
-q

##</code><code>##

>debug packed1
-rcx
CX 0009
:
-d100 l9
0B14:0100  05 00 00 00 62 6A F6 72-6B                        ....bj.rk
-q

>debug packed2
-rcx
CX 0009
:
-d100 l9
0B14:0100  05 00 00 00 62 6A C3 B6-72                        ....bj..r
-q