I don't remember if it was quicker, but there also is the non-looped version:

undef %saw;
@saw{@in} = (1) x @in;
@out = keys %saw;
[download]

I haven't benchmarked it, but I've frequently heard it asserted that even faster is

undef %saw;
@saw{@in} = ();
@out = keys %saw;
[download]

That is, using undef as your values (so you don't need the increment or to create the list of ones).