Recursion Limit

When first using the procedurally-recursive version, warnings were given for deep recursion.
Deep recursion on subroutine "main::sum" at recurse.pl line 17.
To resolve it one can specify no warnings "recursion".
However, the perl debugger still warns about "100 levels deep in subroutine calls!".
To resolve this set the $DB::deep to a large enough value from within the perl debugger.

4.2. Recursion Limit