In general, I think it needlessly invites trouble in terms of readability and maintainability to change types. It'd be pretty easy for another programmer (and, of course, a few months later that could be you) to see that some given scalar variable is a hashref, say, pass it as such... and waste time tracking down a puzzling error because it wasn't.

If one really thought some bit of code really called for it, commenting it'd probably be prudent.