this is documented as an internal function that shouldn't be relied on.

It means you should normally use utf8::encode or Encode::encode 'UTF-8'. 

$ perl -MDevel::Peek -MEncode=_utf8_on,_utf8_off,encode -e'
    _utf8_on( $x = "\342\231\240" );

    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 "\342\231\240"\0 [UTF8 "\x{2660}"]
  PV = 0x81623f0 "\342\231\240"\0
  PV = 0x81920e8 "\342\231\240"\0
  PV = 0x81ff040 "\342\231\240"\0

[download]

But if _utf8_on or equivalent was wrongly used, _utf8_off is appropriate.

In reply to Re^3: deal with incorrectly set utf8 flag by ikegami
in thread deal with incorrectly set utf8 flag by Anonymous Monk

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