most likely your input data is arady in utf8 and you want to convert it a second time to utf8. for example:

use Encode;
my $str = "hi";                # hi is a notmal string.
$str .= chr(0x1234);          # str is now a utf8 string
Encode::decode_utf8($str, 1); # here you get the error.
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Propably you exchange encode and decode. Or the decode function call is not needed in your case.