Renamed file
Don't capture prefix if -prefix isn't used.

Unless the pattern actually allows (or mandates) a prefix, do not
have a capture for one.
Do not capture sign if there cannot be any.

If the pattern should not match signed numbers, that is, because
either of C<< -unsigned => 1 >>, or C<< -sign => undef >>, no C<< sign >>
capture should happen.