As soon as you try making it any valid entity format, you're back to no look behind because its fixed width only http://perldoc.perl.org/perlre.html#%28?%3C!pattern%29

my $f = 'blah;, ente°, blah;, blah.';
$f =~ s{(?<!&[#]x[A-Za-z0-9]{2});,}{,}g;
print "$f\n";
__END__
blah, ente&#xB0;, blah, blah.
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