From utf8 docs:

Note that if you have bytes with the eighth bit on in your script (for example embedded Latin-1 in your string literals), use utf8 will be unhappy since the bytes are most probably not well-formed UTF-8

I guess that, since 192 is "malformed", it is not re-encoded to utf8.

update: From "use encode" docs (perl 5.8 only?):

This pragma also affects encoding of the 0x80..0xFF code point range: normally characters in that range are left as eight-bit bytes (unless they are combined with characters with code points 0x100 or larger, in which case all characters need to become UTF-8 encoded), but if the encoding pragma is present, even the 0x80..0xFF range always gets UTF-8 encoded.