One possible solution is to open CSV something like this:

my $csv = Text::CSV_XS->new ({'always_quote' => 1,
                                 'eol'          => "\n",
                                 'binary'       => 1,
                                });
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That will allow "funny characters" directly in the CSV file.

If you are determined to change the characters to entities, you could do it something like this:

$string=~s/[\x7f-\xff\&]/"&#".ord($1).";"/ge;
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