Looks like you found the caching mechanism. From perlguts: 
  w  PERL_MAGIC_utf8           vtbl_utf8       UTF-8 length+offset cac
+he

[download]

As for keeping bytes... meh, my attachment to the bytes pragma extended only to that use case, as the efficiency of CORE::length() with SVf_UTF8 scalars is a bummer. I'm not even going to bother posting to p5p now that my concern has been addressed another way.

In reply to Re^4: Alternative to bytes::length() by creamygoodness
in thread Alternative to bytes::length() by creamygoodness

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