In both cases you should be manually checking and untainting the data. Taint mode is there for your and your users' protection.

in your example code, you probably want to constrain the allowed paths to place where writing your file won't fubar anything. Or maybe you're happy with users writing files anywhere if privs are elevated. In that case you should make that decision explicit with a fake taint avoidance like in your example code m/(.)/ .

my $.02