sub func_using_comparer(&@) {
   my $cb = shift;

   my $ap = do { no strict 'refs'; \*{caller().'::a'} };
   my $bp = do { no strict 'refs'; \*{caller().'::b'} };

   local *$ap;
   local *$bp;

   ...

   loop {
      ...

      *$ap = \$value1;
      *$bp = \$value2;
      $cb->();

      ...
   };

   ...
}
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