Nitpicking (again): utf8::upgrade and utf8::downgrade do not return converted strings. They do the change inplace, and return a success indicator instead.

One solution is to take a copy (also note that for current Perls, use utf8 isn't needed):

sub verify_upgraded_length {
  my($s) = @_;
  my $u = $s;
  utf8::upgrade($u);
  return length($s) == length($u);
}
sub verify_downgraded_length {
  my($s) = @_;
  my $d = $s;
  utf8::downgrade($d);  # dies if downgrade not possible
  return length($s) == length($d);
}
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