Well, look at that example line this way:

$pi ||= 3;

is like saying:

$pi = $pi || 3;

Now, the || operator short circuits if the first operand is true, so if $pi has any value that is true (i.e. it is not 0, "", or undef -- any others?) then the value of the right hand side will be $pi, otherwise it'll be 3. So, this is a way to say that the default value of $pi is 3, because that's what it'll be set to if it hasn't been set to something else already.

So, in your case, you might do:

$list_info{subscribed_message} ||= $subscribed_message;

Make sense?

Update:This doesn't work, though if "" is a valid value.

So, you could write your assignment as: $list_info{subscribed_message} ||= $subscribed_message; That will set the value of $list_info{subscribed_message} to the value of $subscribed_message, if $list_info{subscribed_message} doesn't exist, or is undef, "", or 0.

if I make a script that says:

 
$pi ||= 3
print $pi;
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I'll get back something like

 
syntax error at or.pl line 1, near "|| ="
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So the way its written won't work

I wonder if this will be different if your testing a hash, since you use exists() instead of defined();

syntax error at or.pl line 1, near "|| ="
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That's a Perl4 (or earlier) error message. Please upgrade to a version of Perl that was released sometime after the Spice Girls hit the pop charts.

-- Randal L. Schwartz, Perl hacker

The code that you provide there is not the code giving you that error message. The error message you list indicates that you have an extraneous space in ||=. ||= should not have any spaces.

The code you provided will give you an entirely different error, namely syntax error at - line 2, near "print" since you didn't have a semicolon at the end of the first line.

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

  initialize_list_info('subscribed_message', $subscribed_message);
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