Is not having it fatal for core operation or can you work around not having it? If not having it is fatal, then you might as well use it so your module fails immediately. If you can work around it then eval the require so you can manage the case where it's missing.

Also, I would tend to think that almost anything that imports subroutines or other symbols really needs to happen at compile time. There are some exceptions, but those tend to be the special cases.

I think newer versions of Carp are lightweight and require the heavy stuff only if invoked. Generally, that pattern may make sense -- require modules at runtime that you only need in a fatal exception (e.g. a dumper). Once the exception is triggered, the loading time of the module at at that point is pretty irrelevant.

I see it kind of like this (not a complete set of permutations, but a set of guidelines):

• Always used in any execution of the program -- use
• Sometimes used, but has import side effects (e.g. importing functions) -- use
• Sometimes used, import not required, but fatal if not present when needed -- use
• Sometimes used, import not required, can work around it not being available -- require (in an eval)
• Sometimes used, specified at runtime (e.g. by a config file) -- require
• Rarely used, fatal if not present, but module is "big" relative to rest of the code -- require or use (personal preference)
• Rarely used, only in fatal error handling -- require