Lexical variables are resolved at compile time so their length is irrelevent beyond the time it takes to read and lex the file. Package variables are a different story. They are essentially hash lookups on the name, and in the case of Fully::Qualified::Package::Names possibily multiple hash lookups. In this case I suspect if you really tortured things you could identify a difference. For most situations you are more likely to be using primarily lexicals and the lengths of your package variabels will have a negligable effect.

The short of it: stick with lexicals and it isn't an issue at all.