When you say Unicode, what encoding do you mean? The most popular and convenient Unicode encoding, UTF-8, and the one Perl likes to use, should be robust against this. Since you are testing for an ASCII character, even if there is non ASCII Unicode data in UTF-8, it would ne passed right through.

If you are dealing with another character encoding altogether, you should use a PerlIO layer to tell Perl what character encoding the data coming in are in.

# If your data are in the UTF-32, which is not ASCII compatible at all
+!
binmode FILE, ":encoding(UTF-32)";
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