#!/usr/bin/env perl
use warnings;
use strict;
use utf8;
use open qw(:utf8 :std);
use Encode qw(decode_utf8);

# ...

if ( not eval{
    # some error-messages (utf8) are decoded some are not
    1 }
) {
    if ( utf8::is_utf8 $@ ) {
        print $@;
    }
    else {
        print decode_utf8( $@ );
    }
}

# $@ is sometimes encoded. If it's not,
# the following will leave it unchanged.
utf8::decode($@);

print $@;