32bit chars? 64bit chars?
32 on 32 bit builds. 64 on 64 bit builds. The format actually allows for 72 bit numbers, but Perl doesn't provide a means of storing and fetching values that large.
UTF 16? UTF 32? UTF 64
Are you asking about the format? It's a variable width format based on UTF-8 confusingly called utf8, but it has nothing to do with Unicode. For starters, the highest possible Unicode character is only 0x10FFFF, far less than what utf8 allows. Unicode has a bunch of reserved and private use and whatnot code points, but not these strings. Unicode imposes certain semantics, but not these strings.