in reply to Re: File ext number
in thread File ext number

If one does not, it will return an undef value.

No, m// never returns undef. (It could, it just doesn't.)

because it doesn't convert to zero.

No, undef DOES convert to zero when used as a number.

In fact, when undef is or would be converted to a string ("") or number (0) is when you get that and similar warnings.