Says nobody in particular:

unless($list_info{subscribed_message} ne "" 
       || exists $list_info{subscribed_message}) {
    $list_info{subscribed_message} = $subscribed_message
}
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The first thing I notice here is that your test is redundant. If x ne "" is true, then exists x must also be true. (Or equivalently, if exists $h{k} is not true, then k is not in the hash, so $h{k} must be equal to "".) So you should rewrite this as:

if ($list_info{subscribed_message} eq "") {
  $list_info{subscribed_message} = $subscribed_message;
}
[download]

which is something of an improvement already.

Now, perhaps this would be a good place to use a mutator subroutine?

  sub set_if_not_set_already {
    $_[0] = $_[1] if $_[0] eq ""; 
  }

  set_if_not_set_already($list_info{subscribed_message},
                         $subscribed_message);
[download]

Or you could abbreviate a little futher and get rid of the mutator:

  sub initialize_list_info {
    $list_info{$_[0]} = $_[1]
        if list_info{$_[0]} eq ""; 
  }

  initialize_list_info('subscribed_message', $subscribed_message);
[download]

This is neither a recommendation nor a disrecommendation. Just something to consider.