The following description was posted on the user's list by Linda Walsh, who did 
the analysis.  It appears to me that she has a pretty decent case for the 
TextWrap problem actually being a small SA bug in the parameters that are 
passed to the module.  The error message started appearing in versions of 
TextWrap that started using the incorrect parameter.

----------------------------

I looked at this error and it appears to be caused by SpamAssassin passing
in an incorrect parameter to "Text::Wrap" by changing the value
of "Text::Wrap::break" to be something other than a "line breaking"
character (or characters).

In file in my Spamassassin-3.17 cpan dir, there is a bad line:

./lib/Mail/SpamAssassin/PerMsgStatus.pm:996:      $hdr = 
Mail::SpamAssassin::Util::wrap($hdr, "\t", "", 79, 0, '(?<=[\s,])');

The last argument, '(?<=[\s,])' appears to be invalid.

The error message is "(?:(?<=[\s,]))* matches null string many \
times in regex; marked by <-- HERE in m/\G(?:(?<=[\s,]))* <-- \
HERE \Z/ at /usr/lib/perl5/5.8.8/Text/Wrap.pm line 46. "

In the Text::Wrap source code of 0704 (last working version),
there is (at line 46, remarkably enough:-)), the line:

        while ($t !~ /\G\s*\Z/gc) {

In versions 0711 and later, that line reads:
        while ($t !~ /\G(?:$break)*\Z/gc) {
=====
    Note that "\s" has been replaced by "(?:$break)".  In the
0711 source code, $break defaults to '\s'.

    In other words -- it appears, from the code it replaces and
from the default values of "$break" that "$break" should contain
a pattern representing the characters to break on.

    However, in PerMsgStatus:996, we see a *zero-length*
(the "(?<=pat)" part) pattern passed in for the value of $break. 
Instead of matching the line "break" character, it only matches
the position and never matches the character itself -- thus it
gets "stuck" applying the zero-length (null pattern) again and
again (thus the message "<bad expr> matches null string many
times....".

    I'm not sure what the author was trying to do in PerMsgStatus.pm
or who "owns" that "line" (or file), but perhaps they meant for
"comma" to be included in the list of "break" characters.  In
which case, instead of:
     '(?<=[\s,])'
for the last argument in line 996, it should be:
     '[\s,]'

    That is, line 996 in lib/Mail/SpamAssassin/PerMsgStatus.pm should be:
$hdr = Mail::SpamAssassin::Util::wrap($hdr, "\t", "", 79, 0, '[\s,]');

(instead of:
$hdr = Mail::SpamAssassin::Util::wrap($hdr, "\t", "", 79, 0, '(?<=[\s,])');
)

I hope this was helpful?

Linda