is_utf8(STRING [, CHECK])
[INTERNAL] Tests whether the UTF8 flag is turned on in the STRING. If CHECK is true, also checks the data in STRING for being well-formed UTF-8. Returns true if successful, false otherwise.
This makes CGI.pm treat all parameters as UTF-8 strings. Use this with care, as it will interfere with the processing of binary uploads. It is better to manually select which fields are expected to return utf-8 strings and convert them using code like this:
my $arg = decode utf8=>param('foo');
Maybe you want Encode::Guess