It is not bad form to leave out the else if you are not using it. In fact I would say that the opposite is the case, adding an unused else is merely cluttering the code.

I would consider you second exapmle to be a mistake, but that's just my opinion.

If you type perlman:perlsyn into the search box It'll bring up the syntax documentation. As you can see the if statement with no else is the first quoted example i.e. it is good style

Nuance