How can I know how many entries are in an associative array?

    While the number of elements in a @foobar array is simply @foobar when
    used in a scalar, you can't figure out how many elements are in an
    associative array in an analogous fashion.  That's because %foobar in
    a scalar context returns the ratio (as a string) of number of buckets
    filled versus the number allocated.  For example, scalar(%ENV) might
    return "20/32".  While perl could in theory keep a count, this would
    break down on associative arrays that have been bound to dbm files.

    However, while you can't get a count this way, one thing you *can* use
    it for is to determine whether there are any elements whatsoever in
    the array, since "if (%table)" is guaranteed to be false if nothing
    has ever been stored in it.  

    So you either have to keep your own count around and increments
    it every time you store a new key in the array, or else do it
    on the fly when you really care, perhaps like this:

        $count++ while each %ENV;

    This preceding method will be faster than extracting the
    keys into a temporary array to count them.

    As of a very recent patch, you can say

        $count = keys %ENV;