Not entirely true - it seems that Smoke is going to just take the code and tweak it - and instead of looping on while (@digits), as thundergnat did, Smoke can loop on reading a single character from File::ReadBackwards at a time instead. At least, that's my impression of what Smoke is going to do.

I still think Smoke is asking for people to write 80% of his code for him, so that he can stamp his other 20% on it and claim it as his own, based solely on his/her comments thus far. But, if we give Smoke the benefit of the doubt for a minute, this is still plausible.

Now, if there only were a File::WriteBackwards available to use, then building $out backwards would be fine, too :-) (Which it seems thundergnat is doing as well, so I'm not sure why that's acceptable.)

Good points.

As for writting, I suppose you could write it out backwards initially. Then, a separate program (or further in the same program) would copy the file in reverse. That can be done as fast as copying it without reversing it.