But if I don't declare %count with my %count% the hash isn't an existing data structure, isn't it?

That's correct (except that it's my %count;, but that's probably just a typo).

Do I always have to declare a hash prior to using it within the foreach loop, or is there a way to do it all in one step within the loop? Sorry if this sounds stupid. ;)

You have to declare any variable that you use, if it's in a for loop or not.

If you want to declare multiple variables in one rush, you can do it like so:

my (%count, @items, $other_stuff, $more_stuff);
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When you get used to it doesn't feel stupid any more to declare variables before you use them ;-)

If you declare a my variable inside a loop, it is only visible inside the loop. So if you wrote:

for my $word (@words){
    my %count; # WRONG
    $count{$word}++;
}
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that would create a variable %count for each iteration, and loose its scope when the iteration finishes.