That's great! I've noticed that more than once when writing my scripts and using -w and, or, use strict, and now I know why. Now those warnings and failed scripts make a whole lot more sense. Thanks for asking a good question!
The intent behind only used once warnings is that it may be a mistyped variable - for instance, you might have a $variable in your code which somewhere is referred to as $vairable. This warning would catch it (however it seems a bit pointless to be warned of single-use variables when at the same type strict will force an error at compile time on mistyped variable names).