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sub sep_id {
...
...
            }
         }
         select STDOUT;

  my $SOME_GLOBAL_VARIABLE = 0;
  some_sub_with_side_effects();
...
  sub some_sub_with_side_effects {
    $SOME_GLOBAL_VARIABLE = 1;
  }

  my $SOME_GLOBAL_VARIABLE = some_sub_without_side_effects();

...
  sub some_sub_without_side_effects {
    return 1;
  }