G'day smartyollie,

There's a semantic issue here. Hashes consist of zero or more pairs of keys and values: talking about a "null hash value" probably isn't what you meant.

I suspect, from the code you've posted, you meant a hash without any key/value pairs. If so, keys will probably do what you want. Here's an example:

#!/usr/bin/env perl -l

use strict;
use warnings;

my %multi = (empty => {}, data => { a => 1 });

for my $key (keys %multi) {
    if (keys %{$multi{$key}}) { 
        print "Data found in $key";
    }
    else {
        print "Data not found in $key";
    }
}
[download]

Output:

$ pm_1129100_complex_hash_check.pl
Data found in data
Data not found in empty
[download]

If I haven't guessed correctly, please read "Checking Variable for Not Null". This was posted last week and contains a discussion about NULL not being a Perl concept, per se. Having done that, please reword your question, replacing "null" with undefined, zero-length, FALSE or whatever you feel is most appropriate.