I think you should be using $subnets{$ip}{'comment'} instead of $subnets{$ip}{$comment} unless the variable $comment contains the string 'comment'.

UPDATE: (before I had seen kennethk's post or he this update:) The whole thing can be expressed as (in newer versions of Perl):

$subnets{$_}{'comment'} //= 'ALLOCATED' for keys %subnets;
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UPDATE 2: Comment from kennethk: code can also be written as

$_->{comment} //= 'ALLOCATED' for values %subnets;
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