The expression "\$$_" is evaluated like a little Perl program and returns a value. In your case, that value seems to be a reference (an object, in fact); the insert method is then invoked on that object.

Personally, I wouldn't really recommend doing things like this unless you're really sure what you're doing. You're basically using symbolic references, since you can also do the same thing w/o the eval. For example:

$foo = "bar";
$_ = "foo";
print $$_;
[download]

This prints "bar".

A better option might be to create a hash of objects, if at all possible. But that's just my opinion.