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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.
The short answer is that you should 3.1.8 and 3.2 and beyond have a work-around to avoid displaying the warning from Text::Wrap. Alternately, you can 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:
http://issues.apache.org/SpamAssassin/show_bug.cgi?id=5056
A possible workaround for this problem is downgrading your For a discussion about downgrading Text::Wrap plugin (part of Perl) discussed here, please see the following thread:
http://www.mail-archive.com/users@spamassassin.apache.org/msg34628.html