What is "$$" for?

[Allen Chan]

Hi all,
I know this is not the place, but what is "$$" for in perl, a line reads $str = "this" ."$$ADS{'name'}";

[Allen Chan]

man, you go extra extra miles for my questions, thanks a ton

[Leslie Chaim]

Allen,
I suppose the closest point to post this would be here
Your question involves Using Hard References. If this is your first encounter to them, it will be a bit tough to grasp them the first time. (I have read it about five times, until they started to make sense, but afterwards its quite rewarding :cool
The '$' in Perl refers to a scalar variable, if there is more then one '$' they are parsed from right to left:
$$$var
is equivalent to ${${${var}}}
In English, you would interoperate that as:
$var is a scalar ref variable(1) which points to an anonymous scalar ref variable(2) which points to another anonymous scalar ref variable(3). Whew, what a mouthful!
So let's focus on:

$$ADS{'name'}

First of all, the quotes around the name KEY is not necessary, unless the KEY has spaces in it, $hash{ KEY } is the same as $hash{KEY}.
From the code, you infer that $ADS is an anonymous hash that is most likely composed on the fly where your reference variable springs into existence (in Perl jargon you say they autovivify). The $ADS hash can also be set explicitly as in:

$ADS = { name   =>  "Bob Smith",
                  title    => "greenhorn",
               }

The {... } is the anonymous hash composer. To access any value you dereference it as $$hashref{KEY}.
You can also use any of these:
$$ADS{name}
${$ADS}{name}
$ADS->{name}
I prefer the last one for its clarity.
Here is some more food for thought (if you have a *nix OS)

perl -le '
$ADS = {
         name =>  "Bob Smith",
         title => "greenhorn",
       };
print ++$i, ":", $ADS;
print ++$i, ":", "\$ADS is ", ref $ADS ? "a reference" : "not a reference";
print ++$i, ":", $ADS{name};
print ++$i, ":", "\$ADS{name} is ", ref $ADS{name} ? "a reference" : "not a reference";
print ++$i, ":", $$ADS{name};
print ++$i, ":", $ADS -> {name};
'

Results in the following:

1:HASH(0x40015c50)
2:$ADS is a reference
3:
4:$ADS{name} is not a reference
5:Bob Smith
6:Bob Smith

Hope this helps�
Cheers,