Actually, looks like utf8::encode is just as dumb as _utf8_off.

$ perl -MDevel::Peek -MEncode=_utf8_on,_utf8_off,encode -e'
    _utf8_on( my $x = "\200\201" );  # Invalid UTF-8

    utf8::encode( my $utf8 = $x );
    my $enc = encode("UTF-8", $x);
    _utf8_off( my $off = $x );

    Dump $x;
    Dump $utf8;
    Dump $enc;
    Dump $off;
'
  PV = 0x8165280 "\200\201"\0 [UTF8 "\x{1}@"]
  PV = 0x81623f0 "\200\201"\0
  PV = 0x81920f8 "\357\277\275\357\277\275"\0
  PV = 0x81ff040 "\200\201"\0
[download]

Except when the flag isn't on.

$ perl -MDevel::Peek -MEncode=_utf8_on,_utf8_off,encode -e'
    my $x = "\200\201";

    utf8::encode( my $utf8 = $x );
    my $enc = encode("UTF-8", $x);
    _utf8_off( my $off = $x );

    Dump $x;
    Dump $utf8;
    Dump $enc;
    Dump $off;
'
  PV = 0x8165278 "\200\201"\0
  PV = 0x816b608 "\302\200\302\201"\0
  PV = 0x8195050 "\302\200\302\201"\0
  PV = 0x81ff038 "\200\201"\0
[download]

Your pick.