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   use Safe;
   my $config = Safe->new;
...
   $ok = $config->reval( $cfg_str );
   # Or: $ok = $config->rdo( $config_source );
   $err = $@;

join ";\n", map "local *$_ = \$handler{$_}", keys %handler;

   local *handler_localizer = sub {
      if( my $name = each %handler ) {
...
         $err = $@;
      }
   };