Consider:

%hash;                 # a hash
$hash{$foo} = 'bar';   # used in scalar context
@ary = qw(foo bar baz);
$hash{@ary}++;         # key is scalar(@ary) here: $hash{3} == 1;
@hash{@ary} = (1,2,3); # hash is now (foo=>1,bar=>2,baz=3)
@hash{@ary}++;         # $hash{baz} == 4; # why?
[download]

What you want is

@hash{@array} = (1) x scalar(@array);
[download]

If you use a hash slice, you use the hash in list context. So assign it a list.

<update> If you say @hash{@ary}++, the hash iterator points to the hash key corresponding to the last element of @ary after the slice has been set up. The value of that key is incremented. </update>

<update> fixed typo (better: bug, negligence) at x - thanks cdarke ;-)
fixed another typo in first code block </update>

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}