I think you might want something like this:

  $SIG{__WARN__}= sub {
        print "\$var= ($var)\n";
        warn @_;
    };
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so that you can see the value of some variable other than the one directly causing the warning (which would show as being undef, of course) in the process of trying to debug this warning.

        - tye (but my friends call me "Tye")