$s1 = "abc\x80";
$s2 = $s1;        # currently SVf_UTF8 not set; string uses 4 bytes of storage
$s2 .= "\x{100}"; # currently perl upgrades to SVf_UTF8 and converts the 0x80 and 0x100 into multi-byte representations
chop($s2);        # currently perl doesn't downgrade; the 0x80 codepoint still stored as 2 bytes
ok($s1 eq $s2);
ok(length($s1) == length($s2));