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package MyEmptyTrueVar;
our $singleton;
...
  cmp  => sub { !ref $_[1] }; # unequal to empty string, or any other 
+string

bless $singleton=\$singleton;

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say "Single str='$MyEmptyTrueVar::singleton', Num=", (0+$MyEmptyTrueVa
+r::singleton)
   if $MyEmptyTrueVar::singleton &&
   $MyEmptyTrueVar::singleton ne '' &&
   '' ne $MyEmptyTrueVar::singleton;