Interesting... Offhand, I can't think of anything, so I'd suggest:

sub equalOrBothNull
{
   my ($a, $b) = @_;

   ((defined($a) && defined($b) && $a==$b) ||
    (!defined($a) && !defined($b));
}
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That way, you can at least hide the problem and make the main code more readable.

By the way, I'm pretty sure

not defined($a) && not defined($b)
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is not what you want.

$ perl -MO=Deparse -e'not defined($a) && not defined($b)'
not defined $a && !defined($b);
-e syntax OK
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Not doesn't bind as tightly as !.
--
Mike