What does that $$ mean? : Firstly, I am beginner in Perl. I would like to know what does this code mean.

    sub function
    {
    	my $var1 = shift;
    	my $var2 = shift;
    
    	if ($$var1)
    	{
    		#code
    	}
    	else
    	{
    		#code
    	}
    }

I know that it is a function, and var1 is first argument of function and var2 is the second argument. But what does mean that $$ in if statement?