Q1) In terms of Mojolicious see Tutorial which introduces Mojolicious::Lite, where Growing takes you from Mojolicious::Lite to a more structured Mojolicious app, which may be overkill depending on your needs. In terms of development, look into version control, something like Git, the docs have some videos giving overviews etc. By using something along these lines you negate the need to keep multiple copies of the same thing (or very similar), with different names in an attempt to keep track of which changes broke what.

Q2) Best practice to try and stick to the same perl and Mojolicious versions.

"The mojo syntax for tables is...idiomatic"

"So now I need to adapt this for Saturn and Jupiter, but I don't want to hard code that part this time: they should be selected. I think I can get this done with 2 sets of radio buttons, if a pull-down menu is more than I want to bite off at this point. So that's where I am tonight."

As mentioned previously these are CSS selectors, $sunrow shows how to get the table row for the Sun. You'd just have to alter this selector based upon values from a drop down (or whatever) to select the details for the rows you wanted. You could even dynamically populate the drop down (or whatever) based on the values from the column in question. That said, if you are looking to host something yourself, I don't see why you're scraping this other site. Wouldn't it make more sense to calculate this stuff yourself? A very helpful Anonymous Monk has given some examples using the Astro namespace modules, Re^2: calculating planet conjunction with mojo front end for example.