I suspect they may be asking for the number of hash elements in a particular hash. If so, perlfaq4 comes to the rescue:

How can I know how many entries are in a hash?

If you mean how many keys, then all you have to do is take the scalar sense of the keys() function:

    $num_keys = scalar keys %hash;
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In void context, the keys() function just resets the iterator, which is faster for tied hashes than would be iterating through the whole hash, one key-value pair at a time.

In other words, it's better to use the above than to use something like these:

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

for (keys %hash) { $number++; }
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However, grep is a good solution if you want something more specific, for example:

## 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);
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