chr(0xFFFF_FFFF) and "\x{10_FFFF}"

I am sitting here writing some test suite scripts for utf8.pm.  I hope
somebody is able to fix bugs as fast I can find them :-)

First two bugs I found:

chr() seems to get things wrong for values bigger than 0x7FFF_FFFF.
Probably some signed/unsigned issue.

  $ perl -Mutf8 -le 'print unpack("H*", chr(0x7FFF_FFFF))'
  fdbfbfbfbfbf
  $ perl -Mutf8 -le 'print unpack("H*", chr(0x8FFF_FFFF))'
  ff

I looks like I can't have _ inside \x{} escapes.

  $ perl -le 'use utf8; print unpack("H*", "\x{10_FFFF}")'
  10

I am using perl5.005_52 on Linux, i586.

Regards,
Gisle