AFAICT, the effect of pack('U0C*', unpack('U0C*', ...)) is the same as utf8::upgrade. See also What does utf8::upgrade actually do.

use warnings;
use strict;
use Devel::Peek;

my @tests = (
    "Hello",
    "Hell\xF6",
    "\N{U+20AC}",
    "\xE2\x82\xAC",
);

for my $in (@tests) {
    my $out = pack('U0C*', unpack('U0C*', $in));
    print STDERR "##### ##### ##### pack/unpack ##### ##### #####\n";
    Dump($in);
    Dump($out);
    print STDERR "##### upgrade #####\n";
    Dump($in);
    utf8::upgrade($in);
    Dump($in);
}
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