I love the functional-programming paradigm that List::Gen brings to Perl. Writing a Collatz sequence with it should be doable, albeit a little challenging since the length of the list is not known a priori.
I'm missing the final 1 at the end of the sequence with the following code:
use List::Gen '*';
iterate{ $_%2 ? 3*$_+1 : $_/2 }->from( 23 )->while( '!=1' )->say;
which prints:
23 70 35 106 53 160 80 40 20 10 5 16 8 4 2
What I essentially need with this approach is a do-while. The documentation makes mention of a while_, which is a 'look-ahead' version of while, but the interpreter cannot find such a method.
Here is a workaround that tests the element for defined-ness to decide when to end the list. It requires modifying the iterator definition to populate an undef element immediately after it encounters a 1 in the chain:
iterate{ $_ == 1 ? undef : $_%2 ? 3*$_+1 : $_/2 }->from( 23 )->while( 'defined' )->say;
which prints
23 70 35 106 53 160 80 40 20 10 5 16 8 4 2 1