It's a bit strange, but the internal representation of the string shouldn't* matter.

What I do find very strange is that it doesn't croak when passed non-bytes.

use strict;
use warnings;

use Data::Dumper qw( Dumper );

$Data::Dumper::Useqq  = 1;
$Data::Dumper::Terse  = 1;
$Data::Dumper::Indent = 0;

my $s = chr(0xC9);
utf8::downgrade($s);
print(Dumper(pack('V/a*', $s)), "\n");
utf8::upgrade($s);
print(Dumper(pack('V/a*', $s)), "\n");

print(Dumper(pack('V/a*', "\x{C9}\x{2660}")), "\n");
[download]

5.10.0:

"\1\0\0\0\311"             # Ok
"\1\0\0\0\x{c9}"           # Ok
"\2\0\0\0\x{c9}\x{2660}"   # Does this make sense???
[download]

On the other hand, 5.8.8 was very broken:

"\1\0\0\0\311"             # Ok
"\1\0\0\0\303"             # XXX
"\2\0\0\0\303\242"         # XXX
[download]