1A/2A for me, pretty much every time. Name the variable what it's supposed to be, then immediately check and skip/abort/throw an exception/halt and catch fire as appropriate if it's actually something else (1A), or reformat it if it's the right thing but not expressed in quite the way you want (2A).

1B and 1C feel like meaningless expansion of code to me. I prefer that subs are short enough to look at the whole thing at once, and "meaningless-but-not-blank" lines of code that don't do anything more than "copy data from an unvalidated-data-name variable into a validated-data-name variable" reduce the number of meaningful lines of code that can be in view, and they don't even give you the visual structure that blank whitespace lines provide.

2B just gives me the heebie-jeebies. Variable names should be meaningful and $arg is the opposite of meaningful. Yes, yes, it is an argument to the sub, but that's the only information the name $arg tells you. I want a name that tells me what the arg is (or at least what it should be). If the only information you want to convey about the value is that it's an argument, you may as well just skip the shift and refer to it as $_[0], or use a bare shift and access it as $_. (Yes, IMO $arg really is that utterly meaningless as a name.)