$num_keys = scalar keys %hash;

##</code><code>##

$number = grep(//, keys %hash);

for (keys %hash) { $number++; }


##</code><code>##


## Find the number of elements that are defined:
$number = grep(defined $hash{$_}, keys %hash);

## Find the number of elements that are "true":
$number = grep($_, values %hash);

## Find the number of elements where the key equals the value:
$number = grep($_ eq $hash{$_}, keys %hash);