First, you're trying to find the length of a UTF-8 string, not that of a Unicode string.

Right now, you are creating that UTF-8 string (by encoding) and printing it at the same time. To find the number its length, you need do something between those two actions. So,

perl -e'
   binmode(STDOUT, ":encoding(UTF-8)";
   $text="\x{00fc}";
   say length($text);
   say $text;
'
[download]

needs to be changed to

perl -MEncode -e'
   $text="\x{00fc}";
   $bytes=encode("UTF-8", $text);
   say encode("UTF-8", length($bytes));
   say $bytes;
'
[download]