Fractional seconds weren't supported by most versions of Unix when Perl was invented, but that has changed for the better over the years. In Perl 6 most times will be represented in floating-point seconds, so a fractional sleep should do what you expect.
Tying and magic are done entirely differently in Perl 6 (through mixins and such), though you can never get entirely away from the fact that some semantics are going to hide other semantics unless you construct a class that specifically knows about both sets of inherited or composed semantics. However, it should work more like you expect, insofar as the latest layer of semantics should hide the built-in magic, and not vice versa.
As for localtime, it was always intended to be a thin interface over C. But Perl 6 will give an OO interface to most of the list-returning functions, and you'll get at the values with methods on the returned object. So we can fix the year+1900 problem at least for the method call.
I'm not brian, but I can probably pass these on to Larry before Perl 6 comes out... :-)