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キーワード教えてもらったんだからマニュアルぐらい読めよ...

Provided that you have specified a character set of ISO-8859-1 (the
default), the standard HTML escaping rules will be used. The "<" char-
acter becomes "<", ">" becomes ">", "&" becomes "&", and the
quote character becomes """. In addition, the hexadecimal 0x8b
and 0x9b characters, which some browsers incorrectly interpret as the
left and right angle-bracket characters, are replaced by their numeric
character entities ("？" and "？"). If you manually change
the charset, either by calling the charset() method explicitly or by
passing a -charset argument to header(), then all characters will be
replaced by their numeric entities, since CGI.pm has no lookup table
for all the possible encodings.

charset()メソッドを使うか、header()に-charset引数を渡せばいいらしい。