silent upgrading situations

I was wondering if there any other good examples of when Perl will
silently upgrade
(as in utf8::upgrade) a string. For instance, perl will do this when
you concatenate
a "non-utf8" string with a utf8 string:

$a = "Hello";     # utf8 flag not set
$b = chr(1024);
$a .= $b;           # $a now has its utf8 flag set
chop $a;           # $a still has utf8 flag ste

Are there any other examples? I'm especially interested in cases where
the non-utf8
string gets upgraded but actually doesn't change.

Thanks,
ER