my $s = "hello";
my $u = "\x{7777}";

print "utf8 flag ", (utf8::is_utf8($s) ? "on":"off"), "\n";  # off

$s .= $u;               # do something which upgrades $s
$s = substr($s, 0, 5);  # get back the orig. "hello"

print "utf8 flag ", (utf8::is_utf8($s) ? "on":"off"), "\n";  # on - $sftp->write(...) would abort