use utf8;
or, on the command line,
perl -Mutf8 -e '$c="f6"; $u=pack("U",hex($c)); print $u, "\x{f6}"' | o
+d -tx1
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In 5.8 you can (and should) specify which encoding perl should use when writing.
By default perl5.8 will assume you want latin1 output (but warn and then output in utf8 those strings that do not fit in latin1).

If you switch your environment to utf8 with something like

env LC_CTYPE=en_US.UTF-8 perl -e ...
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A different way to do it (TIMTOWTDI) is to explicitly switch STDOUT to a different encoding using the newly repurposed binmode():

binmode STDOUT, ':utf8';
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A third way is using the new use open pragma:

use open ':utf8', ':std';
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So to get your one-liner to work, it becomes:

perl -Mopen=:utf8,:std -e '$c="f6"; $u=pack("U",hex($c)); print $u, "\
+x{f6}"' | od -tx1
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I'm sure you'll be able to find your way to the documentation for these new perl5.8 features. A good start is the perluniintro manpage.

Thanks for your good explanation!
I'm developing mainly under MacPerl and need to ask others to check 5.8.

One question still:
How can I use those new options in a compatible way so 5.6 scripts don't choke?

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?

to answer myself: it depends on the LANG environment

[rose@localhost rose]$ export LANG=C    
[rose@localhost rose]$ perl -e 'print "\x{f6}"' | od -tx1
0000000 f6
0000001
[rose@localhost rose]$ perl -e 'print pack "U", 0xf6 ' | od -tx1
0000000 f6
0000001

[rose@localhost rose]$ export LANG=de_DE.UTF-8
[rose@localhost rose]$ perl -e 'print pack "U", 0xf6 ' | od -tx1
0000000 c3 b6
0000002
[rose@localhost rose]$ perl -e 'print "\x{f6}"' | od -tx1
0000000 c3 b6
0000002
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