From your response to the already given answers I understand that this is not what you are looking for but by itself '$$' also refers to the PID of the current process.

use strict;
use warnings;

print "$$\n\n";

sleep( 100 );
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If you run the above code and then run

ps -aef | grep "perl"
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on another window you will see this:

UserName    PID  ...  ... TIME Terminal   TIME <Your code invocation :
+perl whatever.pl>
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NOTE: This has only been tested (by me) on Unix platforms.