use diagnostics, or splain to see expanded error message.

Can't find Unicode property definition "i" at test12.pl line 21, <$fh> line 6 (#1)

(F) You may have tried to use \p which means a Unicode property (for example \p{Lu} is all uppercase letters). If you did mean to use a Unicode property, see perlunicode for the list of known properties. If you didn't mean to use a Unicode property, escape the \p, either by \\p (just the \p) or by \Q\p (the rest of the string, until possible \E).