Why not just switch out the format?

printf +($root_beer ? "%12.4f" : " "x12)."\n", $root_beer;
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That's what I'd do if I didn't care too much about maintainability.

Of course, the correct way to do it is something like this:

if ($root_beer) { printf "%12.4f\n", $root_beer }
else { print " "x12 . "\n"; }
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Or any way that you consider more readable. Where "readable" means that you can look at it 6 months later without thinking, "what the hell was I thinking when I wrote this?" :-)