# encode/decode UTF-8 for files and standard input/output use open qw( :encoding(UTF-8) :std );
pragma changes the default mode of reading and writing text ( files, standard input, standard output, and standard error ) to UTF-8, which is typically what you want when writing new applications.
ASCII is a subset of UTF-8, so this is not expected to cause any problems with legacy ASCII files and will help protect you the accidental file corruption that can happen when treating UTF-8 files as ASCII.
However, it is important that you know what the encoding of your files is that you are dealing with and handle them accordingly. (Reasons that we should not ignore Unicode.) For more in depth treatment of Unicode, please see the Perl Unicode topic.