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This issue is popping up more and more on the users list and in irc. The short version is that newer versions of Text::Wrap (not associated with SpamAssassin) throw this warning for a regular expression that has been in use for years. It's been reported as a bug to the Text::Wrap authors already. More information can be found
The short answer is that you should downgrade your installation of Text::Wrap to an earlier version which doesn't have this issue. This issue was originally reported in SpamAssassin Bugzilla ticket 5056:
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