Have you tried the -C option or, equivalently, the PERL_UNICODE environment variable?
It seems to help, but I don't have enough experience with unicode strings to really test it properly.
$ perl -CSDAL -e 'print "the utf8 flag is ", utf8::is_utf8(shift) ? "o
+n" : "off", " for command-line arguments\n"' hi...the utf8 flag is on
+ for command-line arguments
$ perl -e 'print "the utf8 flag is ", utf8::is_utf8(shift) ? "o
+n" : "off", " for command-line arguments\n"' hi...
the utf8 flag is off for command-line arguments