my declares a new, lexical variable. But you don't want to declare a new variable, you're accessing an item in an existing data structure.

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

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.

I believe the point of "my" declarations is to simplify finding the "root" of a scoped variable. Variable scoping is an effect of command nesting; in your example the "$count{$word}" declaration is nested in the first foreach loop, which is within the scope of the "my $word" declaration and the "my %count" declaration (necessarily added by FunkyMonk). The variables have this scope because they are outside the loop. That means the variables are also available to the next foreach loop. Since the only one used is %count, you could write: foreach (@words) { my $word = $_; and the scope of $word would be limited to the nest where it is actually used.

Unfortunately, niether your nor FunkyMonk's code actually works properly because of "@words=<STDIN>". I am sure what you wanted to do was this:

#!/usr/bin/perl
use warnings;
use strict;
chomp(my $input = <STDIN>);
my @words = split / /,$input;
my %count;
foreach my $word (@words) {  
   $count{$word} += 1;
}
foreach my $key (sort keys %count) { 
   print "$key was found $count{$key} times\n";
}
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