and I don't see that as a bad practice. Works like expected and is documented. The alternative would be to shove the result explicitly through three other subs:

$thingy -> foo (1,2) -> to_output_channel; 
$string = $thingy -> foo (1,2) -> as_string; 
@lines = $thingy -> foo (1,2) -> as_list;
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which gains me what exactly?

It gains you

1. explicit clarity of intent
2. 3 parameterizable output methods
3. 3 methods which can be re-used or overloaded as needed
4. The ability to call all 3 output methods, none or any combination simply by mixing/matching method calls.

Should I avoid localtime and roll my own?

Well, no. Try Date::localtime or Time::Local or anything in the DateTime hierarchy.