What would I have to do to be able to use "s\x{c3}\x{bc}\x{c3}\x{9f}e" as though it were "s\xc3\xbc\xc3\x9fe" ?

I'm not sure what else is going on, but those strings are equivalent.

perl -MO=Deparse -e 'print "s\xc3\xbc\xc3\x9fe"'
perl -MO=Deparse -e 'print "s\x{c3}\x{bc}\x{c3}\x{9f}e"'
[download]

Both parse the same:

print "s\303\274\303\237e";
-e syntax OK
[download]

I wonder if you have standard output encoded correctly. Maybe adding this will help.

binmode STDOUT, ':utf8';
[download]