I don't think there is a general answer which applies everywhere.

I too have old code written in my dark ages, which I never ported. (And contrary to you I already know the possible traps very well.)

IMHO most needed refactoring will imply using explicit my and our declarations for your variables.

I recommend you should make it a habit to write new code with strict and warnings to get used to it.

After a while, maybe a year, you should re-evaluate if it's worth refactoring your old code.

My 2¢