fixup

src/tetras-back

@@ -244,7 +244,7 @@ sub format_log{
     my %args = @_;
     my $date = strftime("%F %X", localtime());
     my $string = "$date\t$args{level}\t$args{message}\n";
-    if( not utf8::is_unicode($string)){
+    if( not utf8::is_utf8($string)){
         utf8::encode($string);
     }
     return $string;