You might try benchmarking the "do_map" case with:

... keys %{ map { $_ => undef }  @array };
                       # not 0
[download]

I actually don't know how it would compare in terms of speed, but it would use a fair bit less memory. (I just checked the process size of "=> 0" vs. "=> undef", and the latter was about 1 MB smaller on a set of 100K hash keys.)