Leading zeros or not, the length is wrong!

That's because "\374" eq "\xfc" eq "\x{00fc}" eq chr(252) eq chr(0xfc) eq chr(0374). That character is one byte long.

Now I see what you mean. Is fits in one byte. But perl uses 2 bytes for it (because this is not ASCII but rather german umlaute):

use Devel::Peek;
use bytes;
$c = pack ("U", 0xfc);
print Dump($c);
print length($c);
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Output:

SV = PV(0x15d559c) at 0x1a45848
  REFCNT = 1
  FLAGS = (POK,pPOK,UTF8)
  PV = 0x15dd39c "\303\274"\0 [UTF8 "\x{fc}"]
  CUR = 2
  LEN = 3
2
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But it seems that representing it as \x{fc} doesn't work, as somebody else already noticed.