Within "perluniintro.pod" I found this:

Note that \x.. (no {} and only two hexadecimal digits), \x{...}, and
chr(...) for arguments less than 0x100 (decimal 256) generate an
eight-bit character for backward compatibility with older Perls.  For
arguments of 0x100 or more, Unicode characters are always produced. If
you want to force the production of Unicode characters regardless of the
numeric value, use pack("U", ...) instead of \x.., \x{...}, or chr().

This explains why "\x{f6}" won't produce utf8 output.
Nonetheless the pack( "U", hex( "f6" ) ) I used originally should spit out a two byte sequence, shouldn't it?