That depends totally on how you are defining "ascii". Even binary files can be considered ASCII because each byte corresponds to an ASCII value without any conversions at all. FTP defines the difference by interpreting OS-dependent newline sequences with "ASCII" files and not with "binary" files.

If you're trying to encode 8-bit data into printable text, you probably want MIME::Base64 or MIME::QuotedPrint. Otherwise you're going to need to be much more specific about the nature of your conversion.