Contemplating it a little bit more myself the past days, I came up with the following reasons:

Simplicity

The way it's written by merlyn is simpler to understand. It is in fact pseudo-code, not real code.

Already an object

The input may already consist of a reference or an object: no need to take a further reference in that case.

Alias / Copy On Write

Perl shouldn't have to copy the contents of $_. You could consider it a bug that it does so now. I'm pretty sure this innefficiency won't exist in Perl 6.

Liz