The best way to do this is via a hash slice.

In the first example the hash values will all be undef*. To assign a value to all the keys you can use the second method:

    my @a = (1..9);
    my %h;

    # hash slice
    @h{@a} = ();

    # Same thing but sets each value to 1
    @h{@a} = (1) x @a;
[download]

Hash, and array, slices are explained in perldata.

--
John.

* It is worth noting that the assignment in the first example is only to the hash value associated with the first element of the array.