in reply to Check/set hash of hashes value

++davido and ++hdb. If you'd like to save yourself a little typing and are using 5.10 or better (probably), you could use Logical Defined Or Assignment Operators to produce what is cleaner and more obvious to me (YMMV): 
foreach my $ip ( keys %subnets ) {
    $subnets{$ip}{comment} //= 'ALLOCATED';
}

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#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.

In Section Seekers of Perl Wisdom