use Encode qw( encode_utf8 );

s{
   ^ U\+ ( [0-9a-fA-F]+ ) \z
}{
   "0x" . uc(unpack("H*", encode_utf8(chr(hex($1)))))
}xe;

##</code><code>##

use Encode qw( decode_utf8 );

s{
   ^ 0x ( (?: [0-9a-fA-F]{2} ){1,4} ) \z
}{
   sprintf("U+%X", ord(decode_utf8(pack("H*", $1))))
}xe;