You can't make core Perl do this
Did I say we should change Perl to allow this? No. Do you understand the premises of this thread? brian was asking for what people consider oddities of Perl thinking. Now I'm sure any oddity has its reason for being odd (after all, it's there the way it is because someone put it there, and not because Larry has a piece of radioactive material next to his computer and he's just watching how the rays mutate Perl) - but that's not the point.
I expect Perl to DWIM (just like it does in many other things), and if on my system select undef, undef, undef, 2.5 waits for about 2500 milliseconds, then there's no reason to assume sleep 2.5 can't.